Group III-V compound semiconductor and group III-V compound semiconductor device using the same

ABSTRACT

An AlGaInP layer is formed on a substrate made of GaAs, and an AlGaAs layer is formed on the AlGaInP layer via a buffer layer therebetween. The buffer layer has a thickness of about 1.1 nm and is made of AlGaInP whose Ga content is smaller than that of the AlGaInP layer. The buffer layer may alternatively be made of AlGaAs whose Al content is smaller than that of the AlGaAs layer.

This application is a divisional of application Ser. No. 10/342,321filed Jan. 15, 2003 now U.S. Pat. No. 6,841,409.

BACKGROUND OF THE INVENTION

A group III-V compound semiconductor is widely used as a material ofvarious semiconductor elements such as a light emitting element, e.g., asemiconductor laser diode (LD) and a light emitting diode (LED), and alogic circuit element, e.g., a field effect transistor (FET) and aheterojunction bipolar transistor (HBT). In these devices, a pluralityof semiconductor layers having different mixed crystal compositions arelayered together to realize intended optical and electricalcharacteristics.

A semiconductor device made of such a group III-V compound semiconductorrequires a thickness control on the nanometer order and an interfacesteepness, and is thus typically manufactured by using a metal organicvapor phase epitaxy (MOVPE) method or a molecular beam epitaxy (MBE)method.

Particularly, Al_(x)Ga_(y)In_(1-x-y)P (where 0≦x≦1, 0≦y≦1, 0≦x+y≦1) andAl_(z)Ga_(1-z)As (where 0≦z≦1), which have band gaps corresponding tothe visible region and the infrared region, respectively, are importantsemiconductor materials of light emitting elements. Furthermore, theband gaps can be varied over a wide range by varying the respective Alcontents, and the electrical and optical characteristics of asemiconductor device using these materials can be variously adjusted bycombining an AlGaInP layer and an AlGaAs layer. In this way, it ispossible to realize a high-performance semiconductor device having a newfunction. Thus, the development of a group III-V compound semiconductorincluding an AlGaInP layer in combination with an AlGaAs layer is veryimportant.

In view of this, the present inventor produced a group III-V compoundsemiconductor as illustrated in FIG. 25 by using an MOVPE method. Asillustrated in FIG. 25, the group III-V compound semiconductor producedby the present inventor includes an AlGaInP layer 202 made ofAl_(0.35)Ga_(0.15)In_(0.5)P having a thickness of about 0.2 μm and anAlGaAs layer 203 made of Al_(0.6)Ga_(0.4)As having a thickness of about0.3 μm, which are deposited in this order on a substrate 201 made ofGaAs.

However, the group III-V compound semiconductor is observed, with humaneyes, to be cloudy across the entire surface thereof, indicatingoccurrence of crystal defects. Further microscopic observation revealsthat there are a large number of crystal defects having a diameter ofabout 1 μm with a density of 1.4×10⁵/cm². This is three to four ordersof magnitude greater than the value obtained when only one layer of theAlGaInP layer 202 or the AlGaAs layer 203 is formed on the substrate 201made of GaAs. This suggests that the occurrence of the crystal defectsis due to the interface between the AlGaInP layer 202 and the AlGaAslayer 203.

Moreover, when the AlGaAs layer 203 is formed on the substrate 201, andthe AlGaInP layer 202 is formed on the AlGaAs layer 203, such crystaldefects are hardly observed. This suggests that the crystal defects arecharacteristic of a structure where the AlGaAs layer 203 is formed onthe AlGaInP layer 202.

The crystal defects occur due to the fact that among interactionsbetween an element of the AlGaInP layer 202 and an element of the AlGaAslayer 203, the interaction between the GaP component and the AlAscomponent is stronger than interactions between other elements orbetween atoms.

Specifically, in the step of growing the AlGaAs layer 203 on the AlGaInPlayer 202 after the AlGaInP layer 202 is formed on the substrate 201,sufficient migration of the AlAs component in the AlGaAs layer 203 beinggrown is inhibited by the interaction between the AlAs component withthe GaP component of the AlGaInP layer 202, thereby disturbing theatomic arrangement at the interface between the AlGaInP layer 202 andthe AlGaAs layer 203.

As described above, when a group III-V compound semiconductor isproduced in which an AlGaAs layer is deposited on an AlGaInP layer, alarge number of crystal defects occur. Therefore, it is difficult torealize a group III-V compound semiconductor, or a group III-V compoundsemiconductor device using the same, in which an AlGaAs layer isdeposited on an AlGaInP layer.

SUMMARY OF THE INVENTION

The present invention has been made in order to solve the problems inthe prior art, and has an object to provide a group III-V compoundsemiconductor device including an AlGaInP layer and an AlGaAs layerformed on the AlGaInP layer, in which the crystal defect density at theinterface between the AlGaInP layer and the AlGaAs layer is reduced.

In order to achieve the object set forth above, a first method formanufacturing a group III-V compound semiconductor device of the presentinvention includes: a first step of forming a first semiconductor layermade of Al_(a)Ga_(b)In_(1-a-b)P (where 0≦a≦1, 0≦b≦1, 0≦a+b≦1); and asecond step of forming a second semiconductor layer made ofAl_(c)Ga_(1-c)As (where 0≦c≦1) on the first semiconductor layer, whereinin the first step, a Ga content b of the first semiconductor layer issmall.

In the first method for manufacturing a group III-V compoundsemiconductor device of the present invention, the first semiconductorlayer has a small Ga content, whereby when the second semiconductorlayer made of AlGaAs is formed on the first semiconductor layer made ofAlGaInP, the crystal defect density at the interface between the firstsemiconductor layer and the second semiconductor layer can be reduced,as compared to a case where the Ga content of the first semiconductorlayer is relatively large, for the following reason. During the growthof the second semiconductor layer on the first semiconductor layer, theinteraction between the GaP component in the first semiconductor layerand the AlAs component in the second semiconductor layer is reduced,thereby facilitating the migration of the AlAs component in the secondsemiconductor layer being formed.

In the first method for manufacturing a group III-V compoundsemiconductor device, it is preferred that the Ga content b in the firstsemiconductor layer is 0.35 or less.

In this way, it is possible to reliably reduce the crystal defectdensity at the interface between the first semiconductor layer and thesecond semiconductor layer.

A first group III-V compound semiconductor device of the presentinvention includes: a first semiconductor layer made ofAl_(a)Ga_(b)In_(1-a-b)P (where 0≦a≦1, 0≦b≦1, 0≦a+b≦1); and a secondsemiconductor layer made of Al_(c)Ga_(1-c)As (where 0≦c≦1) formed on thefirst semiconductor layer, wherein a Ga content b in the firstsemiconductor layer is 0.35 or less.

The first group III-V compound semiconductor device of the presentinvention includes a layered structure of the first semiconductor layermade of AlGaInP and the second semiconductor layer made of AlGaAs, inwhich the Ga content in the first semiconductor layer is 0.35 or less.Therefore, it is possible to reliably reduce the crystal defect densityat the interface between the first semiconductor layer and the secondsemiconductor layer. Thus, it is possible to improve the performance ofa group III-V compound semiconductor device that uses a firstsemiconductor layer and a second semiconductor layer.

It is preferred that the first group III-V compound semiconductor devicefurther includes: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; anactive layer formed on the first cladding layer; and a second claddinglayer made of a compound semiconductor of a second conductivity typeformed on the active layer, wherein the first semiconductor layer isformed in a stripe shape on the second cladding layer.

In this way, in a case where an active layer is designed so that lightis distributed also above a cladding layer in a semiconductor laserdevice in which the second cladding layer and the first semiconductorlayer are used each as a cladding layer of the second conductivity typewhile the second semiconductor layer is used as a part of a contactlayer between the second cladding layer and an electrode, the crystaldefect density does not increase even if the Al content in the secondsemiconductor layer is increased. Therefore, the absorption of light inthe second semiconductor layer is reduced, whereby it is possible toreduce the operating current of the semiconductor laser device.

It is preferred that the first group III-V compound semiconductor devicefurther includes: a first contact layer made of Al_(d)Ga_(1-d)As (where0≦d≦1) of the second conductivity type formed on the secondsemiconductor layer; and a second contact layer made of GaAs of thesecond conductivity type formed on the first contact layer, wherein anAl content d in the first contact layer is larger than an Al content cin the second semiconductor layer.

In this way, the band offset of the valence band between the secondcontact layer and the second semiconductor layer is reduced by the firstcontact layer, whereby it is possible to reduce the operating voltage ofthe semiconductor laser device.

It is preferred that the first group III-V compound semiconductor devicefurther includes: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; anactive layer formed on the first cladding layer; and a second claddinglayer made of a compound semiconductor of a second conductivity typeformed on the active layer, wherein: the first semiconductor layer isprovided on the second cladding layer; and the second semiconductorlayer is formed in a stripe shape.

In this way, in a semiconductor laser device in which the secondcladding layer and the second semiconductor layer are used each as acladding layer of the second conductivity type while the firstsemiconductor layer is used as an etching stop layer for forming thesecond semiconductor layer in a stripe shape, the second semiconductorlayer is made of AlGaAs, which has a high thermal conductivity, wherebyit is possible to improve the property of radiating heat that isgenerated in the active layer and to increase the output power of thegroup III-V compound semiconductor device. Moreover, the firstsemiconductor layer has a relatively small Ga content, whereby it ispossible to form the second semiconductor layer with a reduced crystaldefect density. Therefore, it is possible to reliably obtain a groupIII-V compound semiconductor device having a high output power and ahigh reliability.

In the first group III-V compound semiconductor device, it is preferredthat the second cladding layer is made of Al_(d)Ga_(1-d)As (where 0≦d≦1)of the second conductivity type.

In this way, in addition to the second semiconductor layer, the secondcladding layer is also made of AlGaAs, which has a high thermalconductivity, whereby it is possible to further improve the property ofradiating heat that is generated in the active layer and to furtherincrease the output power of the group III-V compound semiconductordevice.

In the first group III-V compound semiconductor device, it is preferredthat the first cladding layer is made of Al_(d)Ga_(1-d)As (where 0≦d≦1)of the first conductivity type.

In this way, in addition to the second semiconductor layer, the firstcladding layer is also made of AlGaAs, which has a high thermalconductivity, whereby it is possible to further improve the property ofradiating heat that is generated in the active layer and to furtherincrease the output power of the group III-V compound semiconductordevice.

It is preferred that the first group III-V compound semiconductor devicefurther includes: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; and anactive layer formed on the first cladding layer, wherein the firstsemiconductor layer is provided on the active layer.

In this way, in a semiconductor laser device in which the firstsemiconductor layer and the second semiconductor layer are used each asa cladding layer of the second conductivity type, the secondsemiconductor layer is made of AlGaAs, which has a high thermalconductivity, whereby it is possible to improve the property ofradiating heat that is generated in the active layer. Moreover, thefirst semiconductor layer is made of AlGaInP, whereby it is possible toensure sufficient band gap difference with respect to the active layer,and thus to suppress the overflow of carriers from the active layer.Moreover, since the first semiconductor layer has a relatively small Gacontent, the second semiconductor layer can be formed with a reducedcrystal defect density, whereby it is possible to reliably obtain agroup III-V compound semiconductor device having a high performance anda high reliability.

A second group III-V compound semiconductor device of the presentinvention includes: a first semiconductor layer made ofAl_(a)Ga_(b)In_(1-a-b)P (where 0≦a≦1, 0≦b≦1, 0≦a+b≦1); a buffer layermade of Al_(x)Ga_(y)In_(1-x-y)P (where 0≦x≦1, 0≦y≦1, 0x+y≦1) formed onthe first semiconductor layer; and a second semiconductor layer made ofAl_(c)Ga_(1-c)As (where 0≦c≦1) formed on the buffer layer, wherein a Gacontent y in the buffer layer is smaller than a Ga content b in thefirst semiconductor layer.

In the second group III-V compound semiconductor device of the presentinvention, the Ga content in the buffer layer is smaller than that inthe first semiconductor layer, whereby the crystal defect density at theinterface between the second semiconductor layer and the buffer layercan be reduced as compared to a group III-V compound semiconductordevice in which the first semiconductor layer and the secondsemiconductor layer are in direct contact with each other. Thus, it ispossible to improve the performance of a group III-V compoundsemiconductor device that uses a first semiconductor layer and a secondsemiconductor layer.

It is preferred that the second group III-V compound semiconductordevice further includes: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; anactive layer formed on the first cladding layer; and a second claddinglayer made of a compound semiconductor of a second conductivity typeformed on the active layer, wherein: the active layer is a layeredstructure including a plurality of semiconductor layers; the firstsemiconductor layer is provided so as to form an uppermost layer of theactive layer; and the second semiconductor layer is provided under thesecond cladding layer.

In this way, in a semiconductor laser device in which the firstsemiconductor layer is used as an optical guiding layer of the activelayer, even if the impurity concentration of the cladding layer isincreased, the impurity is prevented by the second semiconductor layerfrom diffusing into the active layer. In addition, the secondsemiconductor layer can be formed with a reduced crystal defect density,whereby it is possible to reliably obtain a semiconductor laser devicewith a desirable temperature characteristic.

It is preferred that the second group III-V compound semiconductordevice further includes: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; anactive layer formed on the first cladding layer; and a second claddinglayer made of a compound semiconductor of a second conductivity typeformed on the active layer, wherein the first semiconductor layer isprovided in a stripe shape on the second cladding layer.

In this way, in a case where an active layer is designed so that lightis distributed also above a cladding layer in a semiconductor laserdevice in which the second cladding layer and the first semiconductorlayer are used each as a cladding layer of the second conductivity typewhile the second semiconductor layer is used as a part of a contactlayer between the second cladding layer and an electrode, the crystaldefect density does not increase even if the Al content in the secondsemiconductor layer is increased. Therefore, the absorption of light inthe second semiconductor layer is reduced, whereby it is possible toreduce the operating current of the semiconductor laser device.

It is preferred that the second group III-V compound semiconductordevice further includes: a first contact layer made of Al_(d)Ga_(1-d)As(where 0≦d≦1) of the second conductivity type formed on the secondsemiconductor layer; and a second contact layer made of GaAs of thesecond conductivity type formed on the first contact layer, wherein anAl content d in the first contact layer is larger than an Al content cin the second semiconductor layer.

In this way, the band offset of the valence band between the secondcontact layer and the second semiconductor layer is reduced by the firstcontact layer, whereby it is possible to reduce the operating voltage ofthe semiconductor laser device.

It is preferred that the second group III-V compound semiconductordevice further includes: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; anactive layer formed on the first cladding layer; and a second claddinglayer made of a compound semiconductor of the second conductivity typeformed in a stripe shape on the active layer, wherein the firstsemiconductor layer is provided on the second cladding layer.

In this way, in a semiconductor laser device in which the firstsemiconductor layer and the second semiconductor layer are used each asa contact layer between the second cladding layer and an electrode, theband offset of the valence band between the second cladding layer andthe second semiconductor layer is reduced by the first semiconductorlayer, thereby reducing the operating voltage of the semiconductor laserdevice. Furthermore, the second semiconductor layer having a large Alcontent can be formed with a reduced crystal defect density, whereby itis possible to suppress the absorption of light emitted from the activelayer by the second semiconductor layer.

It is preferred that the second group III-V compound semiconductordevice further includes: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; and anactive layer formed on the first cladding layer, wherein the firstsemiconductor layer is provided on the active layer.

In this way, in a semiconductor laser device in which the firstsemiconductor layer is used as a cladding layer of the secondconductivity type, the second semiconductor layer made of AlGaAs can beformed on the cladding layer with a reduced crystal defect density.

In the second group III-V compound semiconductor device, it is preferredthat: the first semiconductor layer includes a stripe-shaped protrusion;the buffer layer is provided at a foot of the protrusion and on a sidesurface of the protrusion; and the second semiconductor layer containsan impurity of the first conductivity type and is provided with anopening corresponding to a top surface of the protrusion so that thesecond semiconductor layer covers the buffer layer except for an areaover the top surface of the protrusion.

In this way, the second semiconductor layer can be used as a currentblocking layer. Thus, AlGaAs, which has a large band gap, can be usedfor the current blocking layer, whereby it is possible to reduce theloss of light in the current blocking layer.

In the second group III-V compound semiconductor device of the presentinvention, it is preferred that the buffer layer contains an impurity ofthe first conductivity type.

In this way, the buffer layer can be used as a part of a currentblocking layer.

In the second group III-V compound semiconductor device, it is preferredthat the buffer layer is provided on the first semiconductor layer so asto cover the first semiconductor layer including the top surface of theprotrusion.

In the second group III-V compound semiconductor device, it is preferredthat the buffer layer contains an impurity of the second conductivitytype.

In this way, the buffer layer can be used as a part of a cladding layerof the second conductivity type.

It is preferred that the second group III-V compound semiconductordevice further includes: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; anactive layer formed on the first cladding layer; and a second claddinglayer made of a compound semiconductor of a second conductivity typeformed on the active layer, wherein: the first semiconductor layerincludes a stripe-shaped opening and contains an impurity of the firstconductivity type; and the buffer layer is provided on a wall surface ofthe opening in the first semiconductor layer and on an upper surface ofthe first semiconductor layer.

In this way, in a semiconductor laser device in which the firstsemiconductor layer is used as a current blocking layer while the secondsemiconductor layer is used as a cladding layer of the secondconductivity type, the Al content in the second semiconductor layer canbe increased to reduce the loss of light in the cladding layer whilereducing the crystal defect density in the second semiconductor layer.

It is preferred that the second group III-V compound semiconductordevice further includes: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; and anactive layer formed on the first cladding layer, wherein: the activelayer is a layered structure including a plurality of semiconductorlayers; and the first semiconductor layer is provided so as to form anuppermost layer of the active layer.

In this way, in a semiconductor laser device in which the firstsemiconductor layer is used as an optical guiding layer of the activelayer while the second semiconductor layer is used as a cladding layerof the second conductivity type, it is possible to improve the thermalconductivity of the cladding layer of the second conductivity type so asto improve the property of radiating heat that is generated in theactive layer, and thus to increase the output power of the group III-Vcompound semiconductor device.

It is preferred that the second group III-V compound semiconductordevice further includes: an etching stop layer made ofAl_(d)Ga_(c)In_(1-d-e)P (where 0≦d≦1, 0≦e≦1, 0≦d+e≦1) formed on thesecond semiconductor layer; and a second cladding layer made ofAl_(f)Ga_(1-f)As (where 0≦f≦1) of a second conductivity type formed onthe etching stop layer, wherein a Ga content e in the etching stop layeris 0.35 or less.

In this way, the heat radiating property can be improved by using thesecond semiconductor layer and the second cladding layer each as acladding layer of the second conductivity type. In addition, the secondcladding layer can be processed with a high precision and with a reducedcrystal defect density by using the etching stop layer.

It is preferred that the second group III-V compound semiconductordevice further includes: an etching stop layer made ofAl_(d)Ga_(e)In_(1-d-e)P (where 0≦d≦1, 0≦e≦1, 0≦d+e≦1) formed on thesecond semiconductor layer; a third semiconductor layer made ofAl_(f)Ga_(g)In_(1-f-g)P (where 0≦f≦1, 0≦g≦1, 0≦f+g≦1) formed on theetching stop layer; and a second cladding layer made of Al_(h)Ga_(1-h)As(where 0≦h≦1) of a second conductivity type formed on the thirdsemiconductor layer, wherein a Ga content g in the third semiconductorlayer is smaller than a Ga content e in the etching stop layer.

In this way, the second cladding layer can be formed with a reducedcrystal defect density by using the third semiconductor layer as abuffer layer. Therefore, it is possible to obtain a group III-V compoundsemiconductor device with a reduced crystal defect density by reducingthe Al content in the etching stop layer so as to prevent the etchingstop layer from being oxidized during the manufacturing process.

In the second semiconductor laser device, it is preferred that the firstcladding layer is made of Al_(d)Ga_(1-d)As (where 0≦d≦1).

In this way, the thermal conductivity can be increased also in thecladding layer of the first conductivity type, in addition to thecladding layer of the second conductivity type, thereby furtherimproving the heat radiating property.

It is preferred that the second group III-V compound semiconductordevice further includes: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; and anactive layer formed on the first cladding layer, wherein the firstsemiconductor layer is formed on the active layer.

In this way, the first semiconductor layer and the second semiconductorlayer can be formed, with a reduced crystal defect density, each as acladding layer of the second conductivity type. Therefore, the propertyof radiating heat that is generated in the active layer can be improvedby the second semiconductor layer that is made of AlGaAs, which has ahigh thermal conductivity. Moreover, the overflow of carriers from theactive layer can be suppressed by the first semiconductor layer made ofAlGaInP, which has a large band gap.

It is preferred that the second group III-V compound semiconductordevice further includes: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; anactive layer formed on the first cladding layer; and a second claddinglayer made of Al_(d)Ga_(e)In_(1-d-e)P (where 0≦d≦1, 0≦e≦1, 0≦d+e≦1) of asecond conductivity type formed on the active layer, wherein the firstsemiconductor layer is formed on the second cladding layer.

In this way, the property of radiating heat that is generated in theactive layer can be improved by the second semiconductor layer, whilethe overflow of carriers from the active layer can be suppressed by thesecond cladding layer. Furthermore, since the second semiconductor layercan be formed with a reduced crystal defect density by providing thebuffer layer, it is possible to reduce the Al content in the etchingstop layer so as to prevent the etching stop layer from being oxidizedduring the manufacturing process, thereby obtaining a group III-Vcompound semiconductor device with a reduced crystal defect density.

In the second group III-V compound semiconductor device, it is preferredthat a Ga content y in the buffer layer is zero.

In this way, the crystal defect density at the interface between thesecond semiconductor layer and the buffer layer is reduced reliably.

In the second group III-V compound semiconductor device, it is preferredthat: the substrate is made of GaAs; an In content 1-a-b in the firstsemiconductor layer satisfies 0.45≦1-a-b≦0.55; and an In content 1-x-yin the buffer layer satisfies 0.45≦1-x-y≦0.55.

In this way, the buffer layer is lattice-matched with the substrate,whereby it is possible to reliably obtain a group III-V compoundsemiconductor device with a reduced crystal defect density.

In the second group III-V compound semiconductor device, it is preferredthat a thickness of the buffer layer is 0.5 nm or more and 5 nm or less.

In this way, it is possible to reduce the crystal defect density at theinterface between the first semiconductor layer or the secondsemiconductor layer and the buffer layer while the buffer layer givessubstantially no influence on the electrical and optical characteristicsof the group III-V compound semiconductor device.

A second method for manufacturing a group III-V compound semiconductordevice of the present invention includes: a first step of forming afirst semiconductor layer made of Al_(a)Ga_(b)In_(1-a-b)P (where 0≦a≦1,0≦b≦1, 0≦a+b≦1); and a second step of forming a second semiconductorlayer made of Al_(c)Ga_(1-c)As (where 0≦c≦1) on the first semiconductorlayer, wherein in the second step, an Al content c of the secondsemiconductor layer is small.

In the second method for manufacturing a group III-V compoundsemiconductor device of the present invention, the second semiconductorlayer has a small Al content, whereby when the semiconductor layer madeof AlGaAs is formed on the first semiconductor layer made of AlGaInP,the crystal defect density at the interface between the firstsemiconductor layer and the second semiconductor layer can be reduced,as compared to a case where the Al content of the second semiconductorlayer is relatively large, for the following reason. During the growthof the second semiconductor layer on the first semiconductor layer, theinteraction between the GaP component in the first semiconductor layerand the AlAs component in the second semiconductor layer is reduced,thereby facilitating the migration of the AlAs component in the secondsemiconductor layer being formed.

In the second method for manufacturing a group III-V compoundsemiconductor device, it is preferred that the Al content c in thesecond semiconductor layer is 0.3 or less.

In this way, the crystal defect density at the interface between thefirst semiconductor layer and the second semiconductor layer can bereduced reliably.

A third group III-V compound semiconductor device of the presentinvention includes: a first semiconductor layer made ofAl_(a)Ga_(b)In_(1-a-b)P (where 0≦a≦1, 0≦b≦1, 0≦a+b≦1); and a secondsemiconductor layer made of Al_(c)Ga_(1-c)As (where 0≦c≦1) formed on thefirst semiconductor layer, wherein an Al content c in the secondsemiconductor layer is 0.3 or less.

The third group III-V compound semiconductor device of the presentinvention includes a layered structure of the first semiconductor layermade of AlGaInP and the second semiconductor layer made of AlGaAs, inwhich the Al content in the second semiconductor layer is 0.3 or less.Therefore, it is possible to reliably reduce the crystal defect densityat the interface between the first semiconductor layer and the secondsemiconductor layer. Thus, it is possible to improve the performance ofa group III-V compound semiconductor device that uses a firstsemiconductor layer and a second semiconductor layer.

A fourth group III-V compound semiconductor device of the presentinvention includes: a first semiconductor layer made ofAl_(a)Ga_(b)In_(1-a-b)P (where 0≦a≦1, 0≦b≦1, 0≦a+b≦1); a buffer layermade of Al_(z)Ga_(1-z)As (where 0≦z≦1) formed on the first semiconductorlayer; and a second semiconductor layer made of Al_(c)Ga_(1-c)As (where0≦c≦1) formed on the buffer layer, wherein an Al content z in the bufferlayer is smaller than an Al content c in the second semiconductor layer.

In the fourth group III-V compound semiconductor device of the presentinvention, the Al content in the buffer layer is smaller than that inthe second semiconductor layer, whereby the crystal defect density atthe interface between the first semiconductor layer and the buffer layercan be reduced as compared to a group III-V compound semiconductordevice in which the first semiconductor layer and the secondsemiconductor layer are in direct contact with each other. Thus, it ispossible to improve the performance of a group III-V compoundsemiconductor device that uses a first semiconductor layer and a secondsemiconductor layer.

It is preferred that the fourth group III-V compound semiconductordevice further includes an active layer formed on a substrate, theactive layer being a layered structure including a plurality ofsemiconductor layers, wherein: the first semiconductor layer is formedon the substrate; and the second semiconductor layer is provided so asto form a lowermost layer of the active layer.

In this way, in a semiconductor laser device in which the firstsemiconductor layer is used as a cladding layer of the firstconductivity type while the second semiconductor layer is used as anoptical guiding layer of the active layer, it is possible to ensure asufficient band gap difference using a cladding layer of anAlGaInP-based material with an active layer of an AlGaAs-based material.Therefore, it is possible to reliably suppress the overflow of electronsfrom the active layer into the cladding layer, whereby it is possible toreliably obtain a semiconductor laser device having a reduced thresholdcurrent and a desirable temperature characteristic.

In the fourth group III-V compound semiconductor device, it is preferredthat an Al content z in the buffer layer is zero.

In this way, the crystal defect density at the interface between thefirst semiconductor layer and the buffer layer is reduced reliably.

In the fourth group III-V compound semiconductor device, it is preferredthat: the substrate is made of GaAs; and an In content 1-a-b in thefirst semiconductor layer satisfies 0.45≦1-a-b≦0.55.

In this way, the first semiconductor layer is lattice-matched with thesubstrate, whereby it is possible to reliably obtain a group III-Vcompound semiconductor device with a reduced crystal defect density.

In the fourth group III-V compound semiconductor device, it is preferredthat a thickness of the buffer layer is 0.5 nm or more and 5 nm or less.

In this way, it is possible to reduce the crystal defect density at theinterface between the first semiconductor layer or the secondsemiconductor layer and the buffer layer while the buffer layer givessubstantially no influence on the electrical and optical characteristicsof the group III-V compound semiconductor device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the relationship between the Al content xin an Al_(x)Ga_(0.5-x)In_(0.5)P layer and the crystal defect density fora group III-V compound semiconductor device of the present invention, inwhich an Al_(0.6)Ga_(0.4)As layer is deposited on theAl_(x)Ga_(0.5-x)In_(0.5)P layer.

FIG. 2 is a graph illustrating the relationship between the Al content zin an Al_(z)Ga_(1-z)As layer and the crystal defect density for a groupIII-V compound semiconductor device of the present invention, in whichthe Al_(z)Ga_(1-z)As layer is formed on a Ga_(0.5)In_(0.5)P layer.

FIG. 3 is a cross-sectional view illustrating a group III-V compoundsemiconductor layered structure according to a first embodiment of thepresent invention.

FIG. 4 is a cross-sectional view illustrating a group III-V compoundsemiconductor layered structure according to a second embodiment of thepresent invention.

FIG. 5 is a cross-sectional view illustrating a semiconductor laserdevice according to a third embodiment of the present invention.

FIG. 6 is a cross-sectional view illustrating a semiconductor laserdevice according to a fourth embodiment of the present invention.

FIG. 7 is a cross-sectional view illustrating a semiconductor laserdevice according to a fifth embodiment of the present invention.

FIG. 8 is a cross-sectional view illustrating a semiconductor laserdevice according to a first variation of the fifth embodiment of thepresent invention.

FIG. 9 is a cross-sectional view illustrating a semiconductor laserdevice according to a second variation of the fifth embodiment of thepresent invention.

FIG. 10 is a cross-sectional view illustrating a semiconductor laserdevice according to a third variation of the fifth embodiment of thepresent invention.

FIG. 11 is a cross-sectional view illustrating a semiconductor laserdevice according to a fourth variation of the fifth embodiment of thepresent invention.

FIG. 12 is a cross-sectional view illustrating a semiconductor laserdevice according to a sixth embodiment of the present invention.

FIG. 13A to FIG. 13C are cross-sectional views sequentially illustratingsteps in a method for manufacturing a semiconductor laser deviceaccording to the sixth embodiment of the present invention.

FIG. 14 is a cross-sectional view illustrating a semiconductor laserdevice according to a variation of the sixth embodiment of the presentinvention.

FIG. 15 is a cross-sectional view illustrating a semiconductor laserdevice according to a seventh embodiment of the present invention.

FIG. 16A to FIG. 16C are cross-sectional views sequentially illustratingsteps in a method for manufacturing a semiconductor laser deviceaccording to the seventh embodiment of the present invention.

FIG. 17 is a cross-sectional view illustrating a semiconductor laserdevice according to an eighth embodiment of the present invention.

FIG. 18A to FIG. 18C are cross-sectional views sequentially illustratingsteps in a method for manufacturing a semiconductor laser deviceaccording to the eighth embodiment of the present invention.

FIG. 19 is a cross-sectional view illustrating a semiconductor laserdevice according to a variation of the eighth embodiment of the presentinvention.

FIG. 20 is a cross-sectional view illustrating a semiconductor laserdevice according to a ninth embodiment of the present invention.

FIG. 21 is a cross-sectional view illustrating a semiconductor laserdevice according to a first variation of the ninth embodiment of thepresent invention.

FIG. 22 is a cross-sectional view illustrating a semiconductor laserdevice according to a second variation of the ninth embodiment of thepresent invention.

FIG. 23 is a cross-sectional view illustrating a semiconductor laserdevice according to a tenth embodiment of the present invention.

FIG. 24 is a cross-sectional view illustrating a semiconductor laserdevice according to an eleventh embodiment of the present invention.

FIG. 25 is a cross-sectional view illustrating a conventional groupIII-V compound semiconductor.

DETAILED DESCRIPTION OF THE INVENTION

A group III-V compound semiconductor device of the present invention anda method for manufacturing the same will now be described with referenceto the drawings. In the following description, “AlGaInP” is defined as“Al_(x)Ga_(y)In_(1-x-y)P (where 0≦x≦1, 0≦y≦1, 0≦x+y≦1)”, and “AlGaAs” isdefined as “Al_(z)Ga_(1-z)As (where 0≦z≦1)”. Moreover, “GaInP” is usedfor Al_(x)Ga_(y)In_(1-x-y)P where x=0, i.e., Ga_(y)In_(1-y)P (where0≦y≦1), and “AlInP” is used for Al_(x)Ga_(y)In_(1-x-y)P where y=0, i.e.,Al_(x)In_(1-x)P (where 0≦x≦1).

The present inventor has found that in a layered structure of a groupIII-V compound semiconductor in which an AlGaAs layer is deposited on anAlGaInP layer, the crystal defect density is reduced as the Ga contentof the AlGaInP layer is reduced, or as the Al content of the AlGaAslayer is reduced.

The relationship between the Al content in a group III-V compoundsemiconductor in which an AlGaAs layer is deposited on an AlGaInP layerand the crystal defect density will now be described with reference tothe drawings.

FIG. 1 illustrates the relationship between the Al content x in anAl_(x)Ga_(0.5-x)In_(0.5)P layer (where 0≦x≦0.5) and the crystal defectdensity in a case where an Al_(0.6)Ga_(0.4)As layer is formed on theAl_(x)Ga_(0.5-x)In_(0.5)P layer. Herein, “crystal defect density” isdefined as the number of crystal defects per unit area that aremicroscopically observed. Moreover, the In content is set to be 0.5 forthe lattice match with a substrate made of GaAs.

As illustrated in FIG. 1, the crystal defect density is smaller as theAl content x in the Al_(x)Ga_(0.5-x)In_(0.5)P layer is larger.Specifically, while the crystal defect density is about 2.7×10⁷/cm²where x=0, it is reduced to about 4.0×10⁶/cm² where x=0.15, and isfurther reduced to about 1×10⁵/cm² where x=0.35. Furthermore, wherex=0.5, hardly any crystal defects are observed, and the number ofcrystal defects per square centimeter is on the order of 10.

As described above, it can be seen that when an AlGaAs layer is formedon an AlGaInP layer, the crystal defect density is smaller as the Alcontent in the AlGaInP layer is larger, or as the Ga content is smaller.Specifically, the crystal defect density will be sufficiently small ifthe Ga content y in the Al_(x)Ga_(y)In_(1-x-y)P layer is 0.35 or less.

Note that while FIG. 1 illustrates the crystal defect density in a casewhere an Al_(0.6)Ga_(0.4)As layer is formed on anAl_(x)Ga_(0.5-x)In_(0.5)P layer, similar results are obtained also whenthe Al content of the AlGaAs layer formed on theAl_(x)Ga_(0.5-x)In_(0.5)P layer is not 0.6. Thus, the crystal defectdensity is smaller as the Ga content of the AlGaInP layer is smaller,irrespective of the composition of the AlGaAs layer.

FIG. 2 illustrates the relationship between the Al content z in anAl_(z)Ga_(1-z)As layer (where 0≦z≦1) and the crystal defect density in acase where the Al_(z)Ga_(1-z)As layer (where 0≦z≦1) is formed on aGa_(0.5)In_(0.5)P layer.

As illustrated in FIG. 2, the crystal defect density is smaller as theAl content z in the Al_(z)Ga_(1-z)As layer is smaller. Specifically,while the crystal defect density is about 2.7×10⁷/cm² where z=0.6, it isreduced by more than 50% to about 1×10⁷/cm² where z=0.3. Furthermore,where z=0, hardly any crystal defects are observed, and the number ofcrystal defects per square centimeter is on the order of 10.

As described above, it can be seen that when an AlGaAs layer is formedon an AlGaInP layer, the crystal defect density is smaller as the Alcontent in the AlGaAs layer is smaller. Specifically, the crystal defectdensity will be sufficiently small if the Al content z in theAl_(z)Ga_(1-z)As layer is 0.3 or less.

Note that while FIG. 2 illustrates the crystal defect density in a casewhere an Al_(x)Ga_(1-x)As layer is formed on an Ga_(0.5)In_(0.5)P layer,similar results are obtained also when the Al content of the AlGaInPlayer formed under the Al_(x)Ga_(1-x)As layer is not 0. Thus, thecrystal defect density is smaller as the Al content of the AlGaAs layeris smaller, irrespective of the composition of the AlGaInP layer.

As described above, in a group III-V compound semiconductor in which anAlGaAs layer is deposited on an AlGaInP layer, it is possible to reducethe crystal defect density at the interface between the AlGaInP layerand the AlGaAs layer by using an AlGaInP layer having a reduced Gacontent or an AlGaAs layer having a reduced Al content.

It is believed that such an effect is obtained due to the fact thatamong interactions between an element of the AlGaInP layer and anelement of the AlGaAs layer, the interaction between the GaP componentand the AlAs component is stronger than interactions between otherelements or between atoms.

Specifically, in the step of growing the AlGaAs layer on the AlGaInPlayer, as the Ga content in the AlGaInP layer is smaller or as the Alcontent in the AlGaAs layer is smaller, the interaction between the AlAscomponent and the GaP component is smaller, thereby promoting themigration of the AlAs component in the AlGaAs layer being formed, thusreducing the disturbance in the atomic arrangement at the interfacebetween the AlGaInP layer and the AlGaAs layer.

Based on these findings, in the following embodiments of the presentinvention, a second semiconductor layer made of AlGaAs is deposited on afirst semiconductor layer made of AlGaInP, while reducing the Ga contentof the first semiconductor layer or the Al content of the secondsemiconductor layer at the interface between the first semiconductorlayer and the second semiconductor layer, thereby reducing the crystaldefect density at the interface between the first semiconductor layerand the second semiconductor layer.

First Embodiment

The first embodiment of the present invention will now be described withreference to the drawings.

FIG. 3 is a cross-sectional view illustrating a group III-V compoundsemiconductor layered structure according to the first embodiment. Asillustrated in FIG. 3, an AlGaInP layer 12 made ofAl_(0.35)Ga_(0.15)In_(0.5)P having a thickness of about 0.2 μm, a bufferlayer 13 made of Al_(0.5)In_(0.5)P having a thickness of about 1.1 nm,and an AlGaAs layer 14 made of Al_(0.6)Ga_(0.4)As having a thickness ofabout 0.3 μm, are deposited in this order on a substrate 11 made ofGaAs, for example.

A method for manufacturing the group III-V compound semiconductorlayered structure of the first embodiment will now be described.

The group III-V compound semiconductor layered structure of the firstembodiment can be formed by successively growing the semiconductorlayers on the substrate 11 made of GaAs by using an MOVPE method, forexample.

Specifically, triethylgallium (TEG), trimethylaluminum (TMA) andtrimethylindium (TMI) are used as group III compound materials, andphosphine (PH₃) and arsine (AsH₃) are used as group V compoundmaterials. These materials are introduced into a reaction tube made ofquartz with hydrogen as a carrier gas. With the pressure inside thereaction tube being about 1.0×10⁴ Pa (about 76 Torr) and the substratetemperature being about 750° C., materials to be supplied and the amountof each material to be supplied are switched from one to another so asto successively grow the semiconductor layers.

A feature of the first embodiment is that the AlGaAs layer 14 is formedon the AlGaInP layer 12 via the buffer layer 13 made of AlGaInP whose Gacontent is smaller than that of the AlGaInP layer 12. Thus, instead ofdepositing the AlGaAs layer 14 directly on the AlGaInP layer 12 made ofAl_(0.35)Ga_(0.15)In_(0.5)P, the AlGaAs layer 14 is deposited on thebuffer layer 13 made of Al_(0.5)In_(0.5)P whose Ga content is smallerthan that of the AlGaInP layer 12, thereby reducing the crystal defectdensity at the interface between the buffer layer 13 and the AlGaAslayer 14.

Furthermore, since the thickness of the buffer layer 13 is as small asabout 1.1 nm, the buffer layer 13 does not substantially influence theproperties of the group III-V compound semiconductor layered structuresuch as the optical and electrical characteristics thereof.

Thus, in the group III-V compound semiconductor layered structure of thefirst embodiment, the buffer layer 13 made of AlGaInP is interposedbetween the semiconductor layers, whereby it is possible to reduce thecrystal defect density from that of a reference layered structure inwhich the AlGaAs layer 14 is deposited directly on the AlGaInP layer 12,while obtaining substantially the same properties as those of such areference layered structure.

Herein, the AlGaInP compositions of the AlGaInP layer 12 and the bufferlayer 13 are not limited to Al_(0.35)Ga_(0.15)In_(0.5)P andAl_(0.5)In_(0.5)P, respectively, but may alternatively be any othersuitable compositions as long as the Ga content in the buffer layer 13is smaller than that in the AlGaInP layer 12. Of course, the effect ofreducing the crystal defect density is greater as the Ga content in thebuffer layer 13 is smaller.

Moreover, the AlGaAs composition of the AlGaAs layer 14 is not limitedto Al_(0.6)Ga_(0.4)As, but may alternatively be any other suitablecomposition defined as Al_(z)Ga_(1-z)As (where 0≦z≦1).

Moreover, the thickness of the buffer layer 13 is preferably 0.5 nm ormore and 5 nm or less. If it is less than 0.5 nm, the crystal defectdensity at the interface between the buffer layer 13 and the AlGaAslayer 14 reflects the Al content in the AlGaInP layer 12, and the effectof reducing the crystal defect density by the buffer layer 13 isinsufficient. On the other hand, if it is greater than 5 nm, the bufferlayer 13 will give a non-negligible influence on the physical propertiesof the group III-V compound semiconductor layered structure.

In the first embodiment, it is not necessary that the AlGaInP layer 12is in contact with the buffer layer 13. For example, a semiconductorlayer made of AlGaInP whose Ga content is smaller than that of thebuffer layer 13 may alternatively be inserted therebetween. Also in sucha case, it is only required that the Ga content in the buffer layer 13is smaller than that in the AlGaInP layer 12.

Moreover, the layered structure including the AlGaInP layer 12, thebuffer layer 13 and the AlGaAs layer 14 is not required to be formed onthe substrate 11. For example, it may alternatively be formed on anothersemiconductor layer as a part of a layered structure in a semiconductordevice such as a field effect transistor, a heterojunction bipolartransistor, a semiconductor laser device or a light emitting diode.

Moreover, in each of the AlGaInP layer 12 and the buffer layer 13, theIn content is set to be 0.5 for the lattice match with the substrate 11made of GaAs. However, for the purpose of realizing a lattice match withthe substrate 11, the In content, 1-x-y, is not limited to 0.5, but mayalternatively be any other suitable value as long as it satisfies0.45≦1-x-y≦0.55. Moreover, if the lattice match with GaAs is notrequired, the In content, 1-x-y, is only required to satisfy 0≦1-x-y≦1.

As described above, according to the first embodiment, the AlGaAs layer14 is formed on the AlGaInP layer 12 via the buffer layer 13 made ofAlGaInP, and the Ga content in the buffer layer 13 is smaller than thatin the AlGaInP layer 12. In this way, it is possible to obtain a groupIII-V compound semiconductor layered structure in which the crystaldefect density is reduced from that of a reference group III-V compoundsemiconductor layered structure in which the AlGaAs layer 14 isdeposited directly on the AlGaInP layer 12, while obtainingsubstantially the same properties as those of such a reference groupIII-V compound semiconductor layered structure.

Second Embodiment

The second embodiment of the present invention will now be describedwith reference to the drawings.

FIG. 4 is a cross-sectional view illustrating a group III-V compoundsemiconductor layered structure according to the second embodiment. Asillustrated in FIG. 4, an AlGaInP layer 22 made of Ga_(0.5)In_(0.5)Phaving a thickness of about 30 nm, a buffer layer 23 made of GaAs havinga thickness of about 1.6 nm, and an AlGaAs layer 24 made ofAl_(0.6)Ga_(0.4)As having a thickness of about 0.3 μm, are deposited ona substrate 21 made of GaAs.

Such a group III-V compound semiconductor layered structure can beformed by using an MOVPE method, for example, as in the firstembodiment.

A feature of the second embodiment is that the AlGaAs layer 24 is formedon the AlGaInP layer 22 via the buffer layer 23 made of AlGaAs, and theAl content in the buffer layer 23 is smaller than that in the AlGaAslayer 24. Thus, instead of depositing the AlGaAs layer 24 made ofAl_(0.6)Ga_(0.4)As directly on the AlGaInP layer 22, the AlGaAs layer 24is deposited on the buffer layer 23 made of GaAs whose Al content issmaller than that of the AlGaAs layer 24, thereby reducing the crystaldefect density at the interface between the AlGaInP layer 22 and thebuffer layer 23.

Furthermore, since the thickness of the buffer layer 23 is as small asabout 1.6 nm, the buffer layer 23 does not substantially influence theproperties of the group III-V compound semiconductor layered structuresuch as the optical and electrical characteristics thereof.

Thus, in the group III-V compound semiconductor layered structure of thesecond embodiment, the buffer layer 23 made of AlGaAs is interposedbetween the semiconductor layers, whereby it is possible to reduce thecrystal defect density from that of a reference layered structure inwhich the AlGaAs layer 24 is deposited directly on the AlGaInP layer 22,while obtaining substantially the same properties as those of such areference layered structure.

Herein, the AlGaAs compositions of the buffer layer 23 and the AlGaAslayer 24 are not limited to GaAs and Al_(0.6)Ga_(0.4)As, respectively,but may alternatively be any other suitable compositions as long as theAl content in the buffer layer 23 is smaller than that in the AlGaAslayer 24. Of course, the effect of reducing the crystal defect densityis greater as the Al content in the buffer layer 23 is smaller.

Moreover, the AlGaInP composition of the AlGaInP layer 22 is not limitedto Ga_(0.5)In_(0.5)P, but may alternatively be any other suitablecomposition defined as Al_(x)Ga_(y)In_(1-x-y)P (where 0≦x≦1, 0≦y≦1,0≦x+y≦1).

Moreover, the thickness of the buffer layer 23 is preferably 0.5 nm ormore and 5 nm or less. If it is less than 0.5 nm, the crystal defectdensity at the interface between the AlGaInP layer 22 and the bufferlayer 23 reflects the Al content in the AlGaAs layer 24, and the effectof reducing the crystal defect density by the buffer layer 23 isinsufficient. On the other hand, if it is greater than 5 nm, the bufferlayer 23 will give a non-negligible influence on the physical propertiesof the group III-V compound semiconductor layered structure.

In the second embodiment, it is not necessary that the buffer layer 23is in contact with the AlGaAs layer 24. For example, a semiconductorlayer made of AlGaAs whose Al content is smaller than that of the bufferlayer 23 may alternatively be inserted between the buffer layer 23 andthe AlGaAs layer 24. Also in such a case, it is only required that theAl content in the buffer layer 23 is smaller than that in the AlGaAslayer 24.

Moreover, the layered structure including the AlGaInP layer 22, thebuffer layer 23 and the AlGaAs layer 24 is not required to be formed onthe substrate 21. For example, it may alternatively be formed on anothersemiconductor layer as a part of a layered structure in a semiconductordevice such as a field effect transistor, a heterojunction bipolartransistor, a semiconductor laser device or a light emitting diode.

Moreover, in the AlGaInP layer 22, the In content is set to be 0.5 forthe lattice match with the substrate 21 made of GaAs. However, for thepurpose of realizing a lattice match with the substrate 21, the Incontent, 1-x-y, is not limited to 0.5, but may alternatively be anyother suitable value as long as it satisfies 0.45≦1-x-y≦0.55. Moreover,if the lattice match with GaAs is not required, the In content, 1-x-y,is only required to satisfy 0≦1-x-y≦1.

As described above, according to the second embodiment, the AlGaAs layer24 is formed on the AlGaInP layer 22 via the buffer layer 23 made ofAlGaAs, and the Al content in the buffer layer 23 is smaller than thatin the AlGaAs layer 24. In this way, it is possible to obtain a groupIII-V compound semiconductor layered structure in which the crystaldefect density is reduced from that of a reference group III-V compoundsemiconductor layered structure in which the AlGaAs layer 24 isdeposited directly on the AlGaInP layer 22, while obtainingsubstantially the same properties as those of such a reference groupIII-V compound semiconductor layered structure.

Third Embodiment

A semiconductor laser device according to the third embodiment will nowbe described with reference to the drawings.

FIG. 5 is a cross-sectional view illustrating the semiconductor laserdevice according to the third embodiment of the present invention. Asillustrated in FIG. 5, an n-type cladding layer 32 made of n-typeAlGaInP, an active layer 33 including a multiple quantum well andoptical guiding layers interposing the multiple quantum welltherebetween, a buffer layer 34 made of AlGaInP, a dopant diffusionpreventing layer 35 made of AlGaAs, and a first p-type cladding layer 36made of p-type AlGaInP, are deposited in this order on a substrate 31made of n-type GaAs.

Note that in the active layer 33, the multiple quantum well has analternating pattern of three well layers made of GaInP and two barrierlayers made of AlGaInP, with two of the well layers being the outermostlayers of the multiple quantum well, and an optical guiding layer madeof AlGaInP is formed on the upper side and on the lower side of themultiple quantum well.

A second p-type cladding layer 37 made of p-type AlGaInP is formed in astripe shape on the first p-type cladding layer 36. Moreover, a currentblocking layer 38 made of n-type AlGaInP is formed on the upper surfaceof the first p-type cladding layer 36 beside the second p-type claddinglayer 37 and on the side surface of the second p-type cladding layer 37.A contact layer 39 made of p-type GaAs is formed on the second p-typecladding layer 37 and the current blocking layer 38.

An n-side electrode 40 is formed under the substrate 31. The n-sideelectrode 40 is made of an alloy containing Au, Ge and Ni, for example,and is in ohmic contact with the substrate 31. Moreover, a p-sideelectrode 41 is formed on the contact layer 39. The p-side electrode 41is made of an alloy containing Cr, Pt and Au, for example, and is inohmic contact with the contact layer 39.

Herein, the structure is doped with Si as an n-type impurity and Zn as ap-type impurity. Moreover, the doping concentration is substantiallyequal to the carrier concentration at room temperature.

Table 1 below shows, as an example, the specific thickness, compoundcomposition and doping concentration of each of the semiconductorlayers.

TABLE 1 Compound Doping Thickness Composition concentration (cm⁻³)Semiconductor layer Contact layer  4.0 μm GaAs p-type 2 × 10¹⁸ Secondp-type cladding layer  1.0 μm Al_(0.35)Ga_(0.15)In_(0.5)P p-type 1 ×10¹⁸ Current blocking layer  0.3 μm Al_(0.5)In_(0.5)P n-type 1 × 10¹⁸First p-type cladding layer  0.2 μm Al_(0.35)Ga_(0.15)In_(0.5)P p-type 1× 10¹⁸ Dopant diffusion preventing layer   25 nm Al_(0.8)Ga_(0.2)As — —Buffer layer  1.1 nm Al_(0.5)In_(0.5)P — — Active layer Multiple quantumwell Well layers   5 nm each Ga_(0.5)In_(0.5)P — — Barrier layers   6 nmeach Al_(0.25)Ga_(0.25)In_(0.5)P — — Optical guiding layers   25 nm eachAl_(0.25)Ga_(0.25)In_(0.5)P — — N-type cladding layer  1.5 μmAl_(0.35)Ga_(0.15)In_(0.5)P n-type 1 × 10¹⁸ N-type substrate  100 μmGaAs n-type 1 × 10¹⁸

As shown in Table 1 above, the Ga content in the buffer layer 34 issmaller than that in the optical guiding layers. Thus, in thesemiconductor laser device of the third embodiment, the layeredstructure including an optical guiding layer, the buffer layer 34 andthe dopant diffusion preventing layer 35, which are deposited in thisorder, is similar to the group III-V compound semiconductor layeredstructure of the first embodiment.

In the third embodiment, the In content, 1-x-y, in each semiconductorlayer made of Al_(x)Ga_(y)In_(1-x-y)P is set to be 0.5 for the latticematch with the substrate 31 made of GaAs. Note however that for thepurpose of realizing a lattice match with the substrate 31 made of GaAs,the In content, 1-x-y, in each semiconductor layer made ofAl_(x)Ga_(y)In_(1-x-y)P is not limited to 0.5, but may alternatively beany other suitable value as long as it satisfies 0.45≦1-x-y≦0.55.

In the semiconductor laser device of the third embodiment, the activelayer 33 has a multiple quantum well structure with a band gapcorresponding to a wavelength of 650 nm. Therefore, as the currentpasses through the gap in the current blocking layer 38 and reaches theactive layer 33, laser light is emitted with an oscillation wavelengthof 650 nm.

The dopant diffusion preventing layer 35 prevents Zn, which is a dopantof the first p-type cladding layer 36, from diffusing into the activelayer 33 while manufacturing or operating the semiconductor laserdevice. In this way, it is possible to increase the doping concentrationof the first p-type cladding layer 36 without influencing the activelayer 33, whereby it is possible to reliably suppress the overflow ofelectrons from the active layer 33 into the first p-type cladding layer36.

Furthermore, the buffer layer 34 is formed between the active layer 33and the dopant diffusion preventing layer 35. Herein, an optical guidinglayer that is the uppermost layer of the active layer 33 is made ofAl_(0.25)Ga_(0.25)In_(0.5)P, and the buffer layer 34 is made ofAl_(0.5)In_(0.5)P. Thus, the Ga content in the buffer layer 34 issmaller than that in the optical guiding layer, whereby the crystaldefect density at the interface between the buffer layer 34 and thedopant diffusion preventing layer 35 can be reduced from that when thedopant diffusion preventing layer 35 is formed directly on the activelayer 33. Specifically, the number of crystal defects per squarecentimeter at the interface between the buffer layer 34 and the dopantdiffusion preventing layer 35 is on the order of 10.

Herein, the band gap of the buffer layer 34 is transparent to laserlight having an oscillation wavelength of 650 nm, and the thicknessthereof is as small as 1.1 nm, whereby the buffer layer 34 hassubstantially no influence on the optical characteristics (e.g., thedivergence angle) or the electrical characteristics (e.g., the operatingcurrent value) of the semiconductor laser device.

Note that in the third embodiment, the compound composition of thebuffer layer 34 is not limited to Al_(0.5)In_(0.5)P, but mayalternatively be any other suitable composition as long as it is AlGaInPwhose Ga content is smaller than that in the optical guiding layer thatis the uppermost layer of the active layer 33. Moreover, it is notnecessary that the buffer layer 34 is formed directly on the activelayer 33. For example, the buffer layer 34 may be made ofAl_(0.45)Ga_(0.05)In_(0.5)P, while a semiconductor layer made ofAl_(0.5)In_(0.5)P, for example, may be inserted between the active layer33 and the buffer layer 34, and it is still possible to obtain theeffect of reducing the crystal defect density at the interface betweenthe buffer layer 34 and the dopant diffusion preventing layer 35.

As described above, according to the third embodiment, the buffer layer34 is formed between the optical guiding layer that is the uppermostlayer of the active layer 33 and the dopant diffusion preventing layer35, whereby it is possible to form the dopant diffusion preventing layer35 with a reduced crystal defect density. Therefore, even if the dopingconcentration of the first p-type cladding layer 36 is increased, theimpurity will not diffuse into the active layer 33. In this way, it ispossible to reliably suppress the overflow of electrons from the activelayer 33 into the first p-type cladding layer 36, whereby it is possibleto realize, with a reduced defect density, a red semiconductor laserdevice having a desirable temperature characteristic.

Fourth Embodiment

A semiconductor laser device according to the fourth embodiment will nowbe described with reference to the drawings.

FIG. 6 is a cross-sectional view illustrating the semiconductor laserdevice according to the fourth embodiment of the present invention. Asillustrated in FIG. 6, an n-type cladding layer 52 made of n-typeAlGaInP, a buffer layer 53 made of GaAs, an active layer 54 including amultiple quantum well and optical guiding layers interposing themultiple quantum well therebetween, and a first p-type cladding layer 55made of p-type AlGaInP, are deposited in this order on a substrate 51made of n-type GaAs.

Note that in the active layer 54, the multiple quantum well has analternating pattern of two well layers made of GaAs and one barrierlayer made of AlGaAs, with the two well layers being the outermostlayers of the multiple quantum well, and an optical guiding layer madeof AlGaAs is formed on the upper side and on the lower side of themultiple quantum well.

A current blocking layer 56 made of n-type AlGaInP having astripe-shaped opening is formed on the first p-type cladding layer 55,and a second p-type cladding layer 57 made of p-type AlGaInP is formedon the first p-type cladding layer 55 and the current blocking layer 56.A contact layer 58 made of p-type GaAs is formed on the second p-typecladding layer 57.

An n-side electrode 59 is formed under the substrate 51. The n-sideelectrode 59 is made of an alloy containing Au, Ge and Ni, for example,and is in ohmic contact with the substrate 51. Moreover, a p-sideelectrode 60 is formed on the contact layer 58. The p-side electrode 60is made of an alloy containing Cr, Pt and Au, for example, and is inohmic contact with the contact layer 58.

Herein, the structure is doped with Si as an n-type impurity and Zn as ap-type impurity. Moreover, the doping concentration is substantiallyequal to the carrier concentration at room temperature.

Table 2 below shows, as an example, the specific thickness, compoundcomposition and doping concentration of each of the semiconductorlayers.

TABLE 2 Compound Doping Thickness Composition concentration (cm⁻³)Semiconductor layer Contact layer  4.0 μm GaAs p-type 2 × 10¹⁸ Secondp-type cladding layer  1.0 μm Al_(0.35)Ga_(0.15)In_(0.5)P p-type 1 ×10¹⁸ Current blocking layer  0.3 μm Al_(0.5)In_(0.5)P n-type 1 × 10¹⁸First p-type cladding layer  0.2 μm Al_(0.35)Ga_(0.15)In_(0.5)P p-type 5× 10¹⁷ Active layer Multiple quantum well Well layers   4 nm each GaAs —— Barrier layer   8 nm Al_(0.3)Ga_(0.7)As — — Optical guiding layers  15 nm each Al_(0.5)Ga_(0.5)As — — Buffer layer  1.6 nm GaAs — — N-typecladding layer  1.5 μm Al_(0.35)Ga_(0.15)In_(0.5)P n-type 1 × 10¹⁸Substrate  100 μm GaAs n-type 1 × 10¹⁸

As shown in Table 2 above, the Al content in the buffer layer 53 issmaller than that in the optical guiding layers. Thus, in thesemiconductor laser device of the fourth embodiment, the layeredstructure including the n-type cladding layer 52, the buffer layer 53and an optical guiding layer, which are deposited in this order, issimilar to the group III-V compound semiconductor layered structure ofthe second embodiment.

In the fourth embodiment, the In content, 1-x-y, in each semiconductorlayer made of Al_(x)Ga_(y)In_(1-x-y)P is set to be 0.5 for the latticematch with the substrate 51 made of GaAs. Note however that for thepurpose of realizing a lattice match with the substrate 51 made of GaAs,the In content, 1-x-y, in each semiconductor layer made ofAl_(x)Ga_(y)In_(1-x-y)P is not limited to 0.5, but may alternatively beany other suitable value as long as it satisfies 0.45≦1-x-y≦0.55.

In the semiconductor laser device of the fourth embodiment, the activelayer 54 has a multiple quantum well structure with a band gapcorresponding to a wavelength of 780 nm. Therefore, as the currentpasses through the gap in the current blocking layer 56 and reaches theactive layer 54, laser light is emitted with an oscillation wavelengthof 780 nm.

A feature of the fourth embodiment is that the buffer layer 53 isprovided between the active layer 54 and the n-type cladding layer 52,the active layer 54 is made of an AlGaAs-based compound semiconductor,and each of the n-type cladding layer 52, the first p-type claddinglayer 55 and the second p-type cladding layer 57 is made of anAlGaInP-based compound semiconductor.

While an AlGaAs-based material is used for the active layer 54, anAlGaInP-based material having a larger band gap is used for the n-typecladding layer 52, the first p-type cladding layer 55 and the secondp-type cladding layer 57, whereby it is possible to provide a largerband gap difference (band offset) between the active layer and thecladding layers, as compared to a conventional semiconductor laserdevice in which an AlGaAs-based material is used for cladding layers. Inthis way, it is possible to reliably suppress the overflow of electronsfrom the active layer 54 into a cladding layer, whereby it is possibleto reduce the threshold current value and improve the temperaturecharacteristic.

Furthermore, the buffer layer 53 is formed between the n-type claddinglayer 52 and the active layer 54. Herein, the buffer layer 53 is made ofGaAs, and the optical guiding layer is made of Al_(0.5)Ga_(0.5)As. Thus,the Al content in the buffer layer 53 is smaller than that in theoptical guiding layer, whereby the crystal defect density at theinterface between the n-type cladding layer 52 and the buffer layer 53can be reduced from that when the active layer 54 is formed directly onthe n-type cladding layer 52. Specifically, the number of crystaldefects per square centimeter at the interface between the n-typecladding layer 52 and the buffer layer 53 is on the order of 10.

Herein, since the buffer layer 53 has a sufficiently small thickness of1.6 nm, the buffer layer 53 is transparent to light emitted from theactive layer 54 due to the quantum effect, and has substantially noinfluence on the optical characteristics (e.g., the divergence angle) orthe electrical characteristics (e.g., the operating current value) ofthe semiconductor laser device.

Note that in the fourth embodiment, the compound composition of thebuffer layer 53 is not limited to GaAs, but may alternatively be anyother suitable composition as long as it is AlGaAs whose Al content issmaller than that in the optical guiding layer that is the outermostlayer of the active layer 54. Moreover, it is not necessary that thebuffer layer 53 is formed directly on the active layer 54. For example,the buffer layer 53 may be made of Al_(0.1)Ga_(0.9)As, while asemiconductor layer made of GaAs, for example, may be inserted betweenthe buffer layer 53 and the active layer 54, and it is still possible toobtain the effect of reducing the crystal defect density at theinterface between the n-type cladding layer 52 and the buffer layer 53.

As described above, according to the fourth embodiment, the buffer layer53 is formed between the optical guiding layer that is the outermostlayer of the active layer 54 and the n-type cladding layer 52, wherebyit is possible to form an optical guiding layer with a reduced crystaldefect density. It is possible to use a cladding layer made of anAlGaInP-based semiconductor layer for an active layer made of anAlGaAs-based semiconductor layer, and to realize, with a reduced defectdensity, an infrared semiconductor laser device having a reducedthreshold current and a desirable temperature characteristic.

Fifth Embodiment

A semiconductor laser device according to the fifth embodiment will nowbe described with reference to the drawings.

FIG. 7 is a cross-sectional view illustrating the semiconductor laserdevice according to the fifth embodiment of the present invention. Asillustrated in FIG. 7, an n-type cladding layer 62 made of n-typeAlGaInP, an active layer 63 including a multiple quantum well andoptical guiding layers interposing the multiple quantum welltherebetween, a first p-type cladding layer 64 made of p-type AlGaInP,and an etching stop layer 65 made of p-type GaInP, are deposited in thisorder on a substrate 61 made of n-type GaAs.

Note that in the active layer 63, the multiple quantum well has analternating pattern of three well layers made of GaInP and two barrierlayers made of AlGaInP, with two of the well layers being the outermostlayers of the multiple quantum well, and an optical guiding layer madeof AlGaInP is formed on the upper side and on the lower side of themultiple quantum well.

A second p-type cladding layer 66 made of p-type AlGaInP is formed in astripe shape on the etching stop layer 65, and a first contact layer 67made of p-type GaInP is formed on the second p-type cladding layer 66.Moreover, a first current blocking layer 68 made of n-type AlInP isformed on the upper surface of the etching stop layer 65 beside thesecond p-type cladding layer 66 and on the side surface of the secondp-type cladding layer 66. A second current blocking layer 69 made ofn-type GaAs is formed on the first current blocking layer 68.

A buffer layer 70 made of p-type AlInP, a second contact layer 71 madeof p-type AlGaAs and a third contact layer 72 made of p-type GaAs aredeposited in this order on the first contact layer 67 and the secondcurrent blocking layer 69.

An n-side electrode 73 is formed under the substrate 61. The n-sideelectrode 73 is made of an alloy containing Au, Ge and Ni, for example,and is in ohmic contact with the substrate 61. Moreover, a p-sideelectrode 74 is formed on the third contact layer 72. The p-sideelectrode 74 is made of an alloy containing Cr, Pt and Au, for example,and is in ohmic contact with the third contact layer 72.

Herein, the structure is doped with Si as an n-type impurity and Zn as ap-type impurity. Moreover, the doping concentration is substantiallyequal to the carrier concentration at room temperature.

Table 3 below shows, as an example, the specific thickness, compoundcomposition and doping concentration of each of the semiconductorlayers.

TABLE 3 Compound Doping Thickness Composition concentration (cm⁻³)Semiconductor layer Third contact layer  3.5 μm GaAs p-type 2 × 10¹⁸Second contact layer  0.5 μm Al_(0.8)Ga_(0.2)As p-type 2 × 10¹⁸ Bufferlayer  0.6 nm Al_(0.5)In_(0.5)P p-type 1 × 10¹⁸ Second current blockinglayer  0.2 μm GaAs n-type 1 × 10¹⁸ First current blocking layer  0.3 μmAl_(0.5)In_(0.5)P n-type 1 × 10¹⁸ First contact layer   50 nmGa_(0.5)In_(0.5)P p-type 1 × 10¹⁸ Second p-type cladding layer  1.0 μmAl_(0.35)Ga_(0.15)In_(0.5)P p-type 1 × 10¹⁸ Etching stop layer   10 nmGa_(0.5)In_(0.5)P p-type 1 × 10¹⁸ First p-type cladding layer  0.2 μmAl_(0.35)Ga_(0.15)In_(0.5)P p-type 5 × 10¹⁷ Active layer Multiplequantum well Well layers   5 nm each Ga_(0.5)In_(0.5)P — — Barrierlayers   6 nm each Al_(0.25)Ga_(0.25)In_(0.5)P — — Optical guidinglayers   20 nm each Al_(0.25)Ga_(0.25)In_(0.5)P — — N-type claddinglayer  1.5 μm Al_(0.35)Ga_(0.15)In_(0.5)P n-type 1 × 10¹⁸ Substrate  100μm GaAs n-type 1 × 10¹⁸

As shown in Table 3 above, the Ga content in the buffer layer 70 issmaller than that in the first contact layer 67. Thus, in thesemiconductor laser device of the fifth embodiment, the layeredstructure including the first contact layer 67, the buffer layer 70 andthe second contact layer 71, which are deposited in this order, issimilar to the group III-V compound semiconductor layered structure ofthe first embodiment.

In the fifth embodiment, the In content, 1-x-y, in each semiconductorlayer made of Al_(x)Ga_(y)In_(1-x-y)P is set to be 0.5 for the latticematch with the substrate 61 made of GaAs. Note however that for thepurpose of realizing a lattice match with the substrate 61 made of GaAs,the In content, 1-x-y, in each semiconductor layer made ofAl_(x)Ga_(y)In_(1-x-y)P is not limited to 0.5, but may alternatively beany other suitable value as long as it satisfies 0.45≦1-x-y≦0.55.

In the semiconductor laser device of the fifth embodiment, the activelayer 63 has a multiple quantum well structure with a band gapcorresponding to a wavelength of 650 nm. Therefore, as the currentpasses through the gap in the first current blocking layer 68 and thesecond current blocking layer 69 and reaches the active layer 63, laserlight is emitted with an oscillation wavelength of 650 nm.

A feature of the fifth embodiment is the provision of the second contactlayer 71 made of AlGaAs having a large Al content. When thesemiconductor laser device is designed so that laser light isdistributed also above the second p-type cladding layer 66 and the firstcontact layer 67, the second contact layer 71 is transparent to laserlight having an oscillation wavelength of 650 nm, thereby reducing theabsorption of light above the second p-type cladding layer 66. In thisway, it is possible to realize a semiconductor laser device having asmall threshold current and a small operating current.

Furthermore, the buffer layer 70 is formed between the first contactlayer 67 and the second contact layer 71. Herein, the first contactlayer 67 is made of Ga_(0.5)In_(0.5)P, and the buffer layer 70 is madeof Al_(0.5)In_(0.5)P. Thus, the Ga content in the buffer layer 70 issmaller than that in the first contact layer 67, whereby the crystaldefect density at the interface between the buffer layer 70 and thesecond contact layer 71 can be reduced from that when the second contactlayer 71 is formed directly on the first contact layer 67. Specifically,the number of crystal defects per square centimeter at the interfacebetween the buffer layer 70 and the second contact layer 71 is on theorder of 10.

The first contact layer 67 is made of GaInP and has a function ofreducing the potential barrier between the second p-type cladding layer66 and the second contact layer 71. In this way, it is possible toreduce the operating voltage of the semiconductor laser device.

Moreover, the etching stop layer 65 is used to form the second p-typecladding layer 66 in a stripe shape. Specifically, after the firstp-type cladding layer 64, the etching stop layer 65 and a second p-typecladding layer formation film are formed in this order on the activelayer 63, a resist mask is formed on the second p-type cladding layerformation film by using a photolithography method, and the second p-typecladding layer formation film is etched with a mixed solution ofsulfuric acid or tartaric acid and hydrochloric acid. In this process,the second p-type cladding layer formation film made of AlGaInP isetched, whereas the etching stop layer 65 made of GaInP is notsubstantially etched, thereby forming the stripe-shaped second p-typecladding layer 66 from the second p-type cladding layer formation film.

Note that in the fifth embodiment, the compound composition of thebuffer layer 70 is not limited to Al_(0.5)In_(0.5)P, but mayalternatively be any other suitable composition as long as it is AlGaInPwhose Ga content is smaller than that in the first contact layer 67.Moreover, it is not necessary that the buffer layer 70 is formeddirectly on the first contact layer 67. For example, the buffer layer 70may be made of Al_(0.45)Ga_(0.05)In_(0.5)P, while a semiconductor layermade of Al_(0.5)In_(0.5)P, for example, may be inserted between thefirst contact layer 67 and the buffer layer 70, and it is still possibleto obtain the effect of reducing the crystal defect density at theinterface between the second contact layer 71 and the buffer layer 70.

Moreover, the buffer layer 70 is not required to be formed on the firstcontact layer 67 and the second current blocking layer 69, but mayalternatively be formed only on the first contact layer 67.

As described above, according to the fifth embodiment, the buffer layer70 is formed between the first contact layer 67 and the second contactlayer 71, whereby it is possible to form, with a reduced defect density,the second contact layer 71 made of AlGaAs having a band gap that istransparent to light emitted from the active layer 63. Thus, in a casewhere the semiconductor laser device is designed so that laser light isdistributed also above the second p-type cladding layer 66, the loss oflight in the second contact layer 71 is reduced, whereby it is possibleto reduce the threshold current and the operating current of thesemiconductor laser device.

First Variation of Fifth Embodiment

A semiconductor laser device according to the first variation of thefifth embodiment will now be described with reference to the drawings.

FIG. 8 is a cross-sectional view illustrating the semiconductor laserdevice according to the first variation of the fifth embodiment. In FIG.8, like components to those of the semiconductor laser device of thefifth embodiment illustrated in FIG. 7 are denoted by like referencenumerals and will not be further described below.

As illustrated in FIG. 8, the n-type cladding layer 62, the active layer63, the first p-type cladding layer 64, the etching stop layer 65 andthe second p-type cladding layer 66 are deposited in this order on thesubstrate 61. The first current blocking layer 68 and the second currentblocking layer 69 are deposited in this order on the upper surface ofthe etching stop layer 65 beside the second p-type cladding layer 66 andon the side surface of the second p-type cladding layer 66.

Moreover, the second contact layer 71 and the third contact layer 72 aredeposited in this order on the second p-type cladding layer 66 and thesecond current blocking layer 69. The n-side electrode 73 is formedunder the substrate 61, and the p-side electrode 74 is formed on thethird contact layer 72.

The semiconductor laser device of the first variation differs from thesemiconductor laser device of the fifth embodiment in that the firstcontact layer 67 made of GaInP and the buffer layer 70 are not providedon the second p-type cladding layer 66.

A feature of the semiconductor laser device of the first variation isthat it is a layered structure in which the second contact layer 71 madeof AlGaAs is deposited on the second p-type cladding layer 66 made ofAlGaInP in which the Ga content of the second p-type cladding layer 66is relatively small.

Herein, in a layered structure in which a semiconductor layer made ofAlGaAs is deposited on a semiconductor layer made of AlGaInP, thecrystal defect density is smaller as the Ga content of the AlGaInP layeris smaller, as illustrated in FIG. 1. In the first variation, the Gacontent of the second p-type cladding layer 66 is 0.15, and the crystaldefect density at the interface between the second p-type cladding layer66 and the second contact layer 71 is about 1×10⁶/cm². Of course, thecrystal defect density can be further reduced by further reducing the Gacontent of the second p-type cladding layer 66.

Second Variation of Fifth Embodiment

A semiconductor laser device according to the second variation of thefifth embodiment will now be described with reference to the drawings.

FIG. 9 is a cross-sectional view illustrating the semiconductor laserdevice according to the second variation of the fifth embodiment. InFIG. 9, like components to those of the semiconductor laser device ofthe first variation illustrated in FIG. 8 are denoted by like referencenumerals and will not be further described below.

As illustrated in FIG. 9, the semiconductor laser device of the secondvariation differs from the semiconductor laser device of the firstvariation in that a fourth contact layer 75 made of p-typeAl_(0.4)Ga_(0.6)As is formed between the second contact layer 71 and thethird contact layer 72.

Herein, the band offset value of the valence band between the secondcontact layer 71 and the fourth contact layer 75 and that between thefourth contact layer 75 and the third contact layer 72 are 200 meV and230 meV, respectively. On the other hand, when the fourth contact layer75 is not provided, the band offset value of the valence band betweenthe second contact layer 71 and the fourth contact layer is 430 meV.Therefore, in the semiconductor laser device of the second variation,the band offset is reduced by the provision of the fourth contact layer75 as compared to the semiconductor laser device of the first variation.

Note that the AlGaAs composition of the fourth contact layer 75 is notlimited to Al_(0.4)Ga_(0.6)As, but may alternatively be any othersuitable composition as long as it is AlGaAs whose band gap is betweenthose of the second contact layer 71 and the third contact layer 72.Moreover, the band offset value can be further reduced by providing aplurality of AlGaAs layers having gradually varied Al contents betweenthe second contact layer 71 and the third contact layer 72.

Thus, with the semiconductor laser device of the second variation, evenif the Al content of the second contact layer 71 is set to be high sothat the second contact layer 71 is transparent to the wavelength oflight emitted from the active layer 63, it is possible to reduce theband offset of the valence band and thus to reduce the operating voltageby providing an AlGaAs layer whose band gap is between those of thesecond contact layer 71 and the third contact layer 72.

Third Variation of Fifth Embodiment

A semiconductor laser device according to the third variation of thefifth embodiment will now be described with reference to the drawings.

FIG. 10 is a cross-sectional view illustrating the semiconductor laserdevice according to the third variation of the fifth embodiment. In FIG.10, like components to those of the semiconductor laser device of thesecond variation illustrated in FIG. 9 are denoted by like referencenumerals and will not be further described below.

As illustrated in FIG. 10, the semiconductor laser device of the thirdvariation differs from the semiconductor laser device of the secondvariation in that the buffer layer 70 is provided between the secondp-type cladding layer 66 and the second contact layer 71 and between thesecond current blocking layer 69 and the second contact layer 71.

Herein, the composition and the thickness of the buffer layer 70 are thesame as those in the fifth embodiment, and the Ga content therein issmaller than that in the second p-type cladding layer 66. Thus, in thesemiconductor laser device of the third variation, the layered structureincluding the second p-type cladding layer 66, the buffer layer 70 andthe second contact layer 71, which are deposited in this order, issimilar to the group III-V compound semiconductor layered structure ofthe first embodiment.

Therefore, it is possible to reduce the crystal defect density at theinterface between the buffer layer 70 and the second contact layer 71 ascompared to the semiconductor laser devices of the first variation andthe second variation in which the second contact layer 71 is depositeddirectly on the second p-type cladding layer 66.

Moreover, the thickness of the buffer layer 70 is as small as 1.1 nm,whereby the buffer layer 70 has substantially no influence on thecharacteristics of the semiconductor laser device such as the operatingcurrent value, the divergence angle and the operating voltage thereof.

With the semiconductor laser device of the third variation, the crystaldefect density at the interface between the buffer layer 70 and thesecond contact layer 71 is not dependent on the composition of thesecond p-type cladding layer 66. Therefore, it is possible to reduce thecrystal defect density while suitably adjusting the composition of thesecond p-type cladding layer 66, which is a parameter that has asignificant influence on the characteristics of a semiconductor laserdevice such as the vertical divergence angle and the operating currentthereof.

Note that the buffer layer 70 is not required to be formed on the secondcurrent blocking layer 69 as long as it is formed on the second p-typecladding layer 66.

Moreover, the composition of the buffer layer 70 is not limited toAl_(0.5)In_(0.5)P, but may alternatively be any other suitablecomposition as long as it is AlGaInP whose Al content is larger thanthat of the second p-type cladding layer 66.

Fourth Variation of Fifth Embodiment

A semiconductor laser device according to the fourth variation of thefifth embodiment will now be described with reference to the drawings.

FIG. 11 is a cross-sectional view illustrating the semiconductor laserdevice according to the fourth variation of the fifth embodiment. InFIG. 11, like components to those of the semiconductor laser device ofthe third variation illustrated in FIG. 10 are denoted by like referencenumerals and will not be further described below.

As illustrated in FIG. 11, the semiconductor laser device of the fourthvariation differs from the semiconductor laser device of the thirdvariation in that the first contact layer 67 is provided between thesecond p-type cladding layer 66 and the buffer layer 70.

Herein, the composition and the thickness of the first contact layer 67are the same as those in the fifth embodiment, and the Ga content in thebuffer layer 70 is smaller than that in the first contact layer 67.Thus, in the semiconductor laser device of the fourth variation, thelayered structure including the first contact layer 67, the buffer layer70 and the second contact layer 71, which are deposited in this order,is similar to the group III-V compound semiconductor layered structureof the first embodiment, and the crystal defect density at the interfacebetween the first contact layer 67 and the second contact layer 71 isreduced.

Moreover, the semiconductor laser device of the fourth variation has afunction of reducing the potential barrier between the second p-typecladding layer 66 and the second contact layer 71. In this way, it ispossible to reduce the operating voltage of the semiconductor laserdevice.

When the second contact layer 71 is formed directly on the first contactlayer 67, the crystal defect density at the interface between the firstcontact layer 67 and the second contact layer 71 is about 3×10⁷/cm⁻²,and the device deteriorates during its operation, whereby it is notpossible to ensure sufficient reliability of the semiconductor laserdevice. In contrast, in the third variation, the buffer layer 70 isprovided between the first contact layer 67 and the second contact layer71, whereby the number of crystal defects per square centimeter isreduced to be on the order of 10.

As described above, with the semiconductor laser devices of the fifthembodiment and the variations thereof, the second contact layer 71 canbe formed with a reduced crystal defect density by using AlGaAs having aband gap that is transparent to the wavelength of light emitted from theactive layer. Therefore, it is possible to realize a very reliablesemiconductor laser device with a high output power and a high luminancethat does not deteriorate under severe operating environments such asduring a high output power operation at a high temperature.Specifically, these semiconductor laser devices are capable of operatingat an output power as high as 120 mW without saturating its output powereven at an environmental temperature of about 70° C.

Note that in the fifth embodiment and the variations thereof, thematerial of the substrate 61 is not limited to n-type GaAs, but thesubstrate 61 may alternatively be a p-type substrate made of p-typeGaAs, for example.

Moreover, in the semiconductor laser devices of the fifth embodiment andthe variations thereof, a real refractive index waveguide is formed byusing AlInP for the first current blocking layer 68. Alternatively, acomplex refractive index waveguide may be formed by using GaAs for thefirst current blocking layer 68.

Moreover, in the semiconductor laser devices of the fifth embodiment andthe variations thereof, the active layer 63 is not limited to those of amultiple quantum well structure, but may alternatively be an activelayer of a single quantum well structure or a single bulk active layer,for example.

Moreover, in the semiconductor laser devices of the fifth embodiment andthe variations thereof, the waveguide structure is not limited to aridge-shaped waveguide structure, but may alternatively be any othersuitable structure such as an internal stripe-shaped waveguidestructure, and it is still possible to obtain similar effects.

Sixth Embodiment

A semiconductor laser device according to the sixth embodiment will nowbe described with reference to the drawings.

FIG. 12 is a cross-sectional view illustrating the semiconductor laserdevice according to the sixth embodiment of the present invention. Asillustrated in FIG. 12, an n-type cladding layer 82 made of n-typeAlGaInP, an active layer 83 including a multiple quantum well andoptical guiding layers interposing the multiple quantum welltherebetween, and a p-type cladding layer 84 made of p-type AlGaInP andincluding a stripe-shaped protrusion 84 a, are deposited in this orderon a substrate 81 made of n-type GaAs.

Note that in the active layer 83, the multiple quantum well has analternating pattern of three well layers made of GaInP and two barrierlayers made of AlGaInP, with two of the well layers being the outermostlayers of the multiple quantum well, and an optical guiding layer madeof AlGaInP is formed on the upper side and on the lower side of themultiple quantum well.

A buffer layer 85 made of p-type AlGaInP is formed on the p-typecladding layer 84 including the stripe-shaped protrusion 84 a, and acurrent blocking layer 86 made of n-type AlGaAs and including an opening86 a opposing the wall surface of the protrusion 84 a is formed on thebuffer layer 85. A contact layer 87 made of p-type GaAs is formed on thecurrent blocking layer 86 and on a portion of the buffer layer 85 overthe top surface of the protrusion 84 a.

An n-side electrode 88 is formed under the substrate 81. The n-sideelectrode 88 is made of an alloy containing Au, Ge and Ni, for example,and is in ohmic contact with the substrate 81. Moreover, a p-sideelectrode 89 is formed on the contact layer 87. The p-side electrode 89is made of an alloy containing Cr, Pt and Au, for example, and is inohmic contact with the contact layer 87.

Herein, the structure is doped with Si as an n-type impurity and Zn as ap-type impurity. Moreover, the doping concentration is substantiallyequal to the carrier concentration at room temperature.

Among the semiconductor layers described above, the p-type claddinglayer 84 is made of Al_(0.35)Ga_(0.15)In_(0.5)P, the buffer layer 85 ismade of Al_(0.5)In_(0.5)P, and the current blocking layer 86 is made ofAl_(0.9)Ga_(0.1)As. The compositions of the other semiconductor layersmay be the same as those of the third embodiment (Table 1 above).

In the semiconductor laser device of the sixth embodiment, the bufferlayer 85 made of AlGaInP is formed between the p-type cladding layer 84made of AlGaInP and the current blocking layer 86 made of AlGaAs, andthe Ga content in the buffer layer 85 is smaller than that in the p-typecladding layer 84. Thus, in the semiconductor laser device of the sixthembodiment, the layered structure including the p-type cladding layer84, the buffer layer 85 and the current blocking layer 86, which aredeposited in this order, is similar to the group III-V compoundsemiconductor layered structure of the first embodiment.

In the sixth embodiment, the In content, 1-x-y, in each semiconductorlayer made of Al_(x)Ga_(y)In_(1-x-y)P is set to be 0.5 for the latticematch with the substrate 81 made of GaAs. Note however that for thepurpose of realizing a lattice match with the substrate 81 made of GaAs,the In content, 1-x-y, in each semiconductor layer made ofAl_(x)Ga_(y)In_(1-x-y)P is not limited to 0.5, but may alternatively beany other suitable value as long as it satisfies 0.45≦1-x-y≦0.55.

In the semiconductor laser device of the sixth embodiment, the activelayer 83 has a multiple quantum well structure with a band gapcorresponding to a wavelength of 650 nm. Therefore, as the currentpasses through the gap in the current blocking layer 86 and reaches theactive layer 83, laser light is emitted with an oscillation wavelengthof 650 nm.

The current blocking layer 86 is made of AlGaAs whose band gap is largerthan that of the p-type cladding layer 84, and can be made transparentto light emitted from the active layer 83. Therefore, in a case wherethe semiconductor laser device is designed so that laser light isdistributed also above the p-type cladding layer 84, the loss of laserlight in the current blocking layer 86 is reduced, whereby it ispossible to reduce the threshold current and the operating current ofthe semiconductor laser device.

Furthermore, the buffer layer 85 is formed between the p-type claddinglayer 84 and the current blocking layer 86. Herein, the p-type claddinglayer 84 is made of Al_(0.35)Ga_(0.15)In_(0.5)P, and the buffer layer 85is made of Al_(0.5)In_(0.5)P. Thus, the Ga content in the buffer layer85 is smaller than that in the p-type cladding layer 84, whereby thecrystal defect density at the interface between the buffer layer 85 andthe current blocking layer 86 can be reduced from that when the currentblocking layer 86 is formed directly on the p-type cladding layer 84.

Moreover, in the sixth embodiment, the buffer layer 85 is doped with ap-type impurity, and it thus functions as a part of the p-type claddinglayer.

Note that in the sixth embodiment, the compound composition of thebuffer layer 85 is not limited to Al_(0.5)In_(0.5)P, but mayalternatively be any other suitable composition as long as it is AlGaInPwhose Ga content is smaller than that in the p-type cladding layer 84.Moreover, it is not necessary that the buffer layer 85 is formeddirectly on the p-type cladding layer 84. For example, the buffer layer85 may be made of Al_(0.45)Ga_(0.05)In_(0.5)P, while a semiconductorlayer made of Al_(0.5)In_(0.5)P, for example, may be inserted betweenthe p-type cladding layer 84 and the buffer layer 85, and it is stillpossible to obtain the effect of reducing the crystal defect density atthe interface between the buffer layer 85 and the current blocking layer86.

Next, a method for manufacturing the semiconductor laser device of thesixth embodiment having such a structure will now be described withreference to the drawings.

FIG. 13A to FIG. 13C are cross-sectional views sequentially illustratingsteps in a method for manufacturing a semiconductor laser deviceaccording to the sixth embodiment. Note that a portion of the structurebelow the active layer 83 is omitted in FIG. 13B and FIG. 13C, becauseit is as illustrated in FIG. 13A.

First, as illustrated in FIG. 13A, the n-type cladding layer 82 made ofn-type AlGaInP, the active layer 83, and a p-type cladding layerformation layer 84A made of p-type AlGaInP, are grown in this order onthe substrate 81 made of n-type GaAs by using an MOVPE method. Then, astripe-shaped silicon oxide film 90 is formed on the p-type claddinglayer formation layer 84A.

Herein, the active layer 83 is obtained by successively growing thecomponent layers thereof so that a plurality of well layers made ofGaInP and a plurality of barrier layers made of AlGaInP are deposited inan alternating pattern between two optical guiding layers made ofAlGaInP.

Then, as illustrated in FIG. 13B, the p-type cladding layer formationlayer 84A is etched while using the silicon oxide film 90 as a mask toform the p-type cladding layer 84 including the stripe-shaped protrusion84 a, and the silicon oxide film 90 is removed by using hydrogenfluoride. Then, by using an MOVPE method, the buffer layer 85 made ofAlInP and a current blocking layer formation layer 86A made of AlGaAsare grown in this order on the p-type cladding layer 84.

Then, as illustrated in FIG. 13C, the current blocking layer formationlayer 86A is etched until the buffer layer 85 is exposed, thus formingthe current blocking layer 86 including the opening 86 a having a shapeconforming to the shape of the protrusion 84 a.

Then, although not shown, the contact layer 87 is formed on the currentblocking layer 86 and a portion of the buffer layer 85 that is exposedthrough the opening 86 a by using an MOVPE method, after which then-side electrode 88 and the p-side electrode 89 are formed on the lowersurface of the substrate 81 and on the upper surface of the contactlayer 87, respectively, thereby obtaining the semiconductor laser deviceof the sixth embodiment.

As described above, according to the sixth embodiment, the buffer layer85 is formed between the p-type cladding layer 84 and the currentblocking layer 86, whereby it is possible to form, with a reduced defectdensity, the current blocking layer 86 made of AlGaAs having a band gapthat is transparent to light emitted from the active layer 83. Thus, ina case where the semiconductor laser device is designed so that laserlight is distributed also above the p-type cladding layer 84, the lossof laser light in the current blocking layer 86 is reduced, whereby itis possible to reduce the threshold current and the operating current ofthe semiconductor laser device.

Variation of Sixth Embodiment

A semiconductor laser device according to one variation of the sixthembodiment will now be described with reference to the drawings. Thepresent variation differs from the sixth embodiment in that no bufferlayer is formed on the top surface of the protrusion 84 a of the p-typecladding layer 84.

FIG. 14 is a cross-sectional view illustrating the semiconductor laserdevice according to the variation of the sixth embodiment. In FIG. 14,like components to those illustrated in FIG. 12 are denoted by likereference numerals and will not be further described below.

As illustrated in FIG. 14, in the semiconductor laser device of thepresent variation, the p-type cladding layer 84 includes thestripe-shaped protrusion 84 a, and a buffer layer 91 made of p-typeAlGaInP is formed on the wall surface of the protrusion 84 a and on thesurface at the foot of the protrusion 84 a. The current blocking layer86 made of n-type AlGaAs is formed on the buffer layer 91. The contactlayer 87 made of p-type GaAs is formed on the current blocking layer 86and on the top surface of the protrusion 84 a of the p-type claddinglayer 84.

Note that in the present variation, the buffer layer 91 is doped with ap-type impurity, and it thus functions as a part of the p-type claddinglayer. Alternatively, it may be doped with an n-type impurity so that itfunctions as a part of the current blocking layer.

As described above, according to the variation of the sixth embodiment,the following effect can be obtained in addition to those of the sixthembodiment. That is, since the buffer layer 91 is not formed on the topsurface of the protrusion 84 a of the p-type cladding layer 84, thebuffer layer 91 can function as a part of the current blocking layer byintroducing an n-type impurity thereinto or as a part of the p-typecladding layer by introducing a p-type impurity thereinto.

Seventh Embodiment

A semiconductor laser device according to the seventh embodiment willnow be described with reference to the drawings.

FIG. 15 is a cross-sectional view illustrating the semiconductor laserdevice according to the seventh embodiment of the present invention. Asillustrated in FIG. 15, an n-type cladding layer 102 made of n-typeAlGaInP, an active layer 103 including a multiple quantum well andoptical guiding layers interposing the multiple quantum welltherebetween, and a first p-type cladding layer 104 made of p-typeAlGaInP, are deposited in this order on a substrate 101 made of n-typeGaAs.

Note that in the active layer 103, the multiple quantum well has analternating pattern of three well layers made of GaInP and two barrierlayers made of AlGaInP, with two of the well layers being the outermostlayers of the multiple quantum well, and an optical guiding layer madeof AlGaInP is formed on the upper side and on the lower side of themultiple quantum well.

A current blocking layer 105 made of n-type AlGaInP and including astripe-shaped opening 105 a is formed on the first p-type cladding layer104, and a buffer layer 106 made of AlGaInP is formed on the currentblocking layer 105 and on a portion of the first p-type cladding layer104 exposed through the opening 105 a. A second p-type cladding layer107 made of AlGaAs including a downward protrusion 107 a is formed onthe buffer layer 106. The protrusion 107 a has a shape conforming to theshape of the opening 105 a of the current blocking layer 105. A contactlayer 108 made of p-type GaAs is formed on the second p-type claddinglayer 107.

An n-side electrode 109 is formed under the substrate 101. The n-sideelectrode 109 is made of an alloy containing Au, Ge and Ni, for example,and is in ohmic contact with the substrate 101. Moreover, a p-sideelectrode 110 is formed on the contact layer 108. The p-side electrode110 is made of an alloy containing Cr, Pt and Au, for example, and is inohmic contact with the contact layer 108.

Herein, the structure is doped with Si as an n-type impurity and Zn as ap-type impurity. Moreover, the doping concentration is substantiallyequal to the carrier concentration at room temperature.

Among the semiconductor layers described above, the first p-typecladding layer 104 is made of Al_(0.35)Ga_(0.15)In_(0.5)P, the currentblocking layer 105 is made of Al_(0.45)Ga_(0.05)In_(0.5)P, the bufferlayer 106 is made of Al_(0.5)In_(0.5)P, and the second p-type claddinglayer 107 is made of Al_(0.8)Ga_(0.2)As. The compositions of the othersemiconductor layers may be the same as those of the third embodiment(Table 1 above).

In the semiconductor laser device of the seventh embodiment, the bufferlayer 106 made of AlInP is formed between the current blocking layer 105made of AlGaInP and the second p-type cladding layer 107 made of AlGaAs,and the Ga content in the buffer layer 106 is smaller than that in thecurrent blocking layer 105. Thus, in the semiconductor laser device ofthe seventh embodiment, the layered structure including the currentblocking layer 105, the buffer layer 106 and the second p-type claddinglayer is similar to the group III-V compound semiconductor layeredstructure of the first embodiment.

Moreover, in the opening 105 a of the current blocking layer 105, thebuffer layer 106 is formed between the first p-type cladding layer 104made of AlGaInP and the second p-type cladding layer 107, and the Gacontent in the buffer layer 106 is smaller than that in the first p-typecladding layer 104. Thus, in the semiconductor laser device of theseventh embodiment, the layered structure including the first p-typecladding layer 104, the buffer layer 106 and the second p-type claddinglayer 107 is also similar to the group III-V compound semiconductorlayered structure of the first embodiment.

In the seventh embodiment, the In content, 1-x-y, in each semiconductorlayer made of Al_(x)Ga_(y)In_(1-x-y)P is set to be 0.5 for the latticematch with the substrate 101 made of GaAs. Note however that for thepurpose of realizing a lattice match with the substrate 101 made ofGaAs, the In content, 1-x-y, in each semiconductor layer made ofAl_(x)Ga_(y)In_(1-x-y)P is not limited to 0.5, but may alternatively beany other suitable value as long as it satisfies 0.45≦1-x-y≦0.55.

In the semiconductor laser device of the seventh embodiment, the activelayer 103 has a multiple quantum well structure with a band gapcorresponding to a wavelength of 650 nm. Therefore, as the currentpasses through the gap in the current blocking layer 105 and reaches theactive layer 103, laser light is emitted with an oscillation wavelengthof 650 nm.

The buffer layer 106 is doped with a p-type impurity, and it thusfunctions as a part of the p-type cladding layer.

The second p-type cladding layer 107 has a band gap such that it istransparent to light emitted from the active layer 103. Thus, in a casewhere the semiconductor laser device is designed so that laser light isdistributed also above the first p-type cladding layer 104, the loss oflaser light in second p-type cladding layer 107 is reduced, whereby itis possible to reduce the threshold current and the operating current ofthe semiconductor laser device.

Furthermore, the buffer layer 106 is formed between the current blockinglayer 105 and the second p-type cladding layer 107. Herein, the currentblocking layer 105 is made of Al_(0.45)Ga_(0.05)In_(0.5)P, and thebuffer layer 106 is made of Al_(0.5)In_(0.5)P. Thus, the Ga content inthe buffer layer 106 is smaller than that in the current blocking layer105, whereby the crystal defect density at the interface between thebuffer layer 106 and the second p-type cladding layer 107 can be reducedfrom that when the second p-type cladding layer 107 is formed directlyon the current blocking layer 105.

Moreover, in the opening 105 a of the current blocking layer 105, thebuffer layer 106 is formed between the first p-type cladding layer 104and the second p-type cladding layer 107. Herein, the first p-typecladding layer 104 is made of Al_(0.35)Ga_(0.15)In_(0.5)P, and thebuffer layer 106 is made of Al_(0.5)In_(0.5)P. Thus, the Ga content inthe buffer layer 106 is smaller than that in the first p-type claddinglayer 104, whereby the crystal defect density at the interface betweenthe buffer layer 106 and the second p-type cladding layer 107 can bereduced from that when the second p-type cladding layer 107 is formeddirectly on the first p-type cladding layer 104.

Note that the compound composition of the buffer layer 106 is notlimited to Al_(0.5)In_(0.5)P, but may alternatively be any othersuitable composition as long as it is AlGaInP whose Ga content issmaller than that in the current blocking layer 105. Moreover, it is notnecessary that the buffer layer 106 is formed directly on the currentblocking layer 105. For example, the buffer layer 106 may be made ofAl_(0.45)Ga_(0.05)In_(0.5)P, while a semiconductor layer made ofAl_(0.5)In_(0.5)P, for example, may be inserted between the currentblocking layer 105 and the buffer layer 106, and it is still possible toobtain the effect of reducing the crystal defect density at theinterface between the buffer layer 106 and the second p-type claddinglayer 107.

Next, a method for manufacturing the semiconductor laser device of theseventh embodiment having such a structure will now be described withreference to the drawings.

FIG. 16A to FIG. 16C are cross-sectional views sequentially illustratingsteps in a method for manufacturing a semiconductor laser deviceaccording to the seventh embodiment. Note that a portion of thestructure below the active layer 103 is omitted in FIG. 16B and FIG.16C, because it is as illustrated in FIG. 16A.

First, as illustrated in FIG. 16A, the n-type cladding layer 102 made ofn-type AlGaInP, the active layer 103, the first p-type cladding layer104 made of p-type AlGaInP, and a current blocking layer formation layer105A made of n-type AlGaInP, are grown in this order on the substrate101 made of n-type GaAs by using an MOVPE method. Then, a resist mask111 with a stripe-shaped opening patterned therein is formed on thecurrent blocking layer formation layer 105A by using a photolithographymethod.

Herein, the active layer 103 is obtained by successively growing thecomponent layers thereof so that a plurality of well layers made ofGaInP and a plurality of barrier layers made of AlGaInP are deposited inan alternating pattern between two optical guiding layers made ofAlGaInP.

Then, as illustrated in FIG. 16B, the current blocking layer formationlayer 105A is etched while using the resist mask 111 as a mask to formthe current blocking layer 105 including the stripe-shaped opening 105a, and the resist mask 111 is removed by using acetone.

Then, as illustrated in FIG. 16C, the buffer layer 106 made of AlInP,the second p-type cladding layer 107 made of AlGaAs, and the contactlayer 108 made of p-type GaAs, are grown in this order on the currentblocking layer 105 by using an MOVPE method.

Then, although not shown, the n-side electrode 109 and the p-sideelectrode 110 are formed on the lower surface of the substrate 101 andon the upper surface of the contact layer 108, respectively, therebyobtaining the semiconductor laser device of the seventh embodiment.

As described above, according to the seventh embodiment, the bufferlayer 106 is formed between the first p-type cladding layer 104 and thesecond p-type cladding layer 107 and between the current blocking layer105 and the second p-type cladding layer 107, whereby it is possible toform, with a reduced defect density, the second p-type cladding layer107 made of AlGaAs having a band gap that is transparent to lightemitted from the active layer 103. Thus, in a case where thesemiconductor laser device is designed so that laser light isdistributed also above the first p-type cladding layer 104, the loss oflight in the second p-type cladding layer 107 is reduced, whereby it ispossible to reduce the threshold current and the operating current ofthe semiconductor laser device.

Eighth Embodiment

A semiconductor laser device according to the eighth embodiment will nowbe described with reference to the drawings.

FIG. 17 is a cross-sectional view illustrating the semiconductor laserdevice according to the eighth embodiment of the present invention. Asillustrated in FIG. 17, an n-type cladding layer 122 made of n-typeAlGaInP, an active layer 123 including a multiple quantum well andoptical guiding layers interposing the multiple quantum welltherebetween, a first buffer layer 124 made of p-type AlGaInP, a firstp-type cladding layer 125 made of p-type AlGaAs, and an etching stoplayer 126 made of p-type AlGaInP, are deposited in this order on asubstrate 121 made of n-type GaAs.

Note that in the active layer 123, the multiple quantum well has analternating pattern of three well layers made of GaInP and two barrierlayers made of AlGaInP, with two of the well layers being the outermostlayers of the multiple quantum well, and an optical guiding layer madeof AlGaInP is formed on the upper side and on the lower side of themultiple quantum well.

A second p-type cladding layer 127 made of p-type AlGaInP is formed in astripe shape on the etching stop layer 126, and a first contact layer128 made of p-type GaInP is formed on the second p-type cladding layer127. Moreover, a first current blocking layer 129 made of n-type AlInPand a second current blocking layer 130 made of n-type GaAs are formedin this order on the upper surface of the etching stop layer 126 besidethe second p-type cladding layer 127 and on the side surface of thesecond p-type cladding layer 127. Moreover, a second contact layer 131made of p-type GaAs is formed on the first contact layer 128 and thesecond current blocking layer 130.

An n-side electrode 132 is formed under the substrate 121. The n-sideelectrode 132 is made of an alloy containing Au, Ge and Ni, for example,and is in ohmic contact with the substrate 121. Moreover, a p-sideelectrode 133 is formed on the second contact layer 131. The p-sideelectrode 133 is made of an alloy containing Cr, Pt and Au, for example,and is in ohmic contact with the second contact layer 131.

Herein, the structure is doped with Si as an n-type impurity and Zn as ap-type impurity. Moreover, the doping concentration is substantiallyequal to the carrier concentration at room temperature.

Table 4 below shows, as an example, the specific thickness, compoundcomposition and doping concentration of each of the semiconductorlayers.

TABLE 4 Compound Doping Thickness Composition concentration (cm⁻³)Semiconductor layer Second contact layer   3 nm GaAs p-type 2 × 10¹⁸Second current blocking layer  0.2 μm GaAs n-type 1 × 10¹⁸ First currentblocking layer  0.3 μm Al_(0.5)In_(0.5)P n-type 1 × 10¹⁸ First contactlayer   50 nm Ga_(0.5)In_(0.5)P p-type 1 × 10¹⁸ Second p-type claddinglayer  1.0 μm Al_(0.8)Ga_(0.2)As p-type 1 × 10¹⁸ Etching stop layer   10nm Al_(0.5)In_(0.5)P p-type 1 × 10¹⁸ First p-type cladding layer  0.2 μmAl_(0.8)Ga_(0.2)As p-type 5 × 10¹⁷ First buffer layer  1.0 nmAl_(0.5)In_(0.5)P — — Active layer Multiple quantum well Well layers   6nm each Ga_(0.5)In_(0.5)P — — Barrier layers   5 nm eachAl_(0.25)Ga_(0.25)In_(0.5)P — — Optical guiding layers   20 nm eachAl_(0.25)Ga_(0.25)In_(0.5)P — — N-type cladding layer  2.0 μmAl_(0.35)Ga_(0.15)In_(0.5)P n-type 1 × 10¹⁸ Substrate  100 μm GaAsn-type 1 × 10¹⁸

As shown in Table 4 above, the Ga content in the first buffer layer 124is smaller than that in the optical guiding layer that is the uppermostlayer of the active layer 123. Thus, in the semiconductor laser deviceof the, eighth embodiment, the layered structure including the opticalguiding layer, the first buffer layer 124 and the first p-type claddinglayer 125, which are deposited in this order, is similar to the groupIII-V compound semiconductor layered structure of the first embodiment.

Furthermore, the Ga content of the etching stop layer 126 is zero, sothat in the layered structure in which the second p-type cladding layer127 made of AlGaAs is deposited on the etching stop layer 126 made ofAlGaInP, the Ga content of the semiconductor layer made of AlGaInP isrelatively small.

Moreover, in the eighth embodiment, the In content, 1-x-y, in eachsemiconductor layer made of Al_(x)Ga_(y)In_(1-x-y)P is set to be 0.5 forthe lattice match with the substrate 121 made of GaAs. Note however thatfor the purpose of realizing a lattice match with the substrate 121 madeof GaAs, the In content, 1-x-y, in each semiconductor layer made ofAl_(x)Ga_(y)In_(1-x-y)P is not limited to 0.5, but may alternatively beany other suitable value as long as it satisfies 0.45≦1-x-y≦0.55.

In the semiconductor laser device of the eighth embodiment, the activelayer 123 has a multiple quantum well structure with a band gapcorresponding to a wavelength of 650 nm. Therefore, as the currentpasses through the gap in the first current blocking layer 129 and thesecond current blocking layer 130 and reaches the active layer 123,laser light is emitted with an oscillation wavelength of 650 nm.

Herein, in the semiconductor laser device of the eighth embodiment, anAlGaAs-based semiconductor material, which has a high thermalconductivity, is used for the first p-type cladding layer 125 and thesecond p-type cladding layer 127. Therefore, the active layer 123 can beconnected to a heat sink (not shown) via at least the first p-typecladding layer 125 and the second p-type cladding layer 127 so that heatgenerated in the active layer 123 can be efficiently radiated into theheat sink to suppress the increase in the temperature of the activelayer 123. In this way, even if the current value is increased in orderto increase the output power of the device, the light-emittingefficiency does not decrease due to thermal saturation. Thus, it ispossible to increase the output power of the semiconductor laser device.

Furthermore, the Ga content in the first buffer layer 124 is smallerthan that of the optical guiding layer that is the uppermost layer ofthe active layer 123, and the Ga content of the etching stop layer 126is set to a relatively small value, whereby it is possible to reduce thecrystal defect density at the interface between the first buffer layer124 and the first p-type cladding layer 125 and at the interface betweenthe etching stop layer 126 and the second p-type cladding layer 127.

Specifically, when the first p-type cladding layer 125 is formeddirectly on the active layer 123 without using the first buffer layer124, the crystal defect density is 1×10⁶/cm² or more. In contrast, inthe eighth embodiment, the number of crystal defects per squarecentimeter at the interface between the first buffer layer 124 and thefirst p-type cladding layer 125 is on the order of 10, indicating thatthe device performance is improved through the reduction of the crystaldefect density.

Moreover, Al_(0.5)In_(0.5)P is used for the etching stop layer 126,whereby the number of crystal defects per square centimeter at theinterface between the etching stop layer 126 and the second p-typecladding layer 127 is on the order of 10.

Note that in the eighth embodiment, the compound composition of thefirst buffer layer 124 is not limited to Al_(0.5)In_(0.5)P, but mayalternatively be any other suitable composition as long as it is AlGaInPwhose Ga content is smaller than that in the optical guiding layer thatis the uppermost layer of the active layer 123. For example, if the Gacontent is 0.15 or less, the crystal defect density at the interfacebetween the first buffer layer 124 and the first p-type cladding layer125 is as low as about 1×10⁵/cm² or less.

Moreover, the thickness of the first buffer layer 124 is not limited toabout 1 nm. As long as it is 0.5 nm or more and 5 nm or less, it ispossible to obtain the effect of reducing the crystal defect densitywithout influencing the characteristics of the semiconductor laserdevice.

Moreover, the compound composition of the etching stop layer 126 is notlimited to Al_(0.5)In_(0.5)P, but may alternatively be any othersuitable composition as long as it is AlGaInP whose Ga content isrelatively small. For example, with Al_(0.35)Ga_(0.15)In_(0.5)P, thecrystal defect density at the interface between the etching stop layer126 and the second p-type cladding layer 127 is as low as about1×10⁵/cm² or less.

Next, a method for manufacturing the semiconductor laser device of theeighth embodiment having such a structure will now be described withreference to the drawings.

FIG. 18A to FIG. 18C are cross-sectional views sequentially illustratingsteps in a method for manufacturing a semiconductor laser deviceaccording to the eighth embodiment. Note that a portion of the structurebelow the active layer 123 is omitted in FIG. 18B and FIG. 18C, becauseit is as illustrated in FIG. 18A.

First, as illustrated in FIG. 18A, the n-type cladding layer 122 made ofn-type AlGaInP, the active layer 123, the first buffer layer 124 made ofp-type AlGaInP, the first p-type cladding layer 125 made of p-typeAlGaAs, the etching stop layer 126 made of p-type AlGaInP, a secondp-type cladding layer formation layer 127A made of p-type AlGaAs, afirst contact layer formation layer 128A made of p-type GaInP, and aprotection layer 134 made of GaAs, are deposited in this order on thesubstrate 121 made of n-type GaAs by using an MOCVD method or an MBEmethod, for example.

Herein, the active layer 123 is obtained by successively growing thecomponent layers thereof so that a plurality of well layers made ofGaInP and a plurality of barrier layers made of AlGaInP are deposited inan alternating pattern between two optical guiding layers made ofAlGaInP.

Then, as illustrated in FIG. 18B, the protection layer 134 is removed,after which a silicon oxide film 135 is deposited on the first contactlayer formation layer 128A by using a CVD method, for example, and thedeposited silicon oxide film 135 is patterned into a stripe shape byusing a photolithography method and an etching method. Then, while usingthe patterned silicon oxide film 135 as a mask, the first contact layerformation layer 128A and the second p-type cladding layer formationlayer 127A are successively etched to form the first contact layer 128and the second p-type cladding layer 127, respectively, which arestripe-shaped.

Note that the silicon oxide film 135 can be patterned by using hydrogenfluoride, for example. Moreover, the first contact layer formation layer128A may be etched by using a hydrochloric acid-type etchant. In thisway, the first contact layer formation layer 128A can be etchedselectively with respect to the second p-type cladding layer formationlayer 127A.

Moreover, the second p-type cladding layer formation layer 127A may beetched by using an etchant that has a high selectivity ratio for AlGaAswith respect to AlGaInP, e.g., a mixture of ammonia and hydrogenperoxide. In this way, the etching process substantially stops at theetching stop layer 126 made of AlGaInP, whereby the second p-typecladding layer 127 can be processed with a good controllability.

Herein, in a case where an AlGaAs-based semiconductor material is usedfor the etching stop layer 126, it is necessary to ensure some etchingselectivity ratio by giving a substantial difference between the Alcontent of the second p-type cladding layer 127 and that of the etchingstop layer 126. However, when the Al content of an AlGaAs-basedsemiconductor material is small, it is not possible to ensure a band gapthat is transparent to the oscillation wavelength of the active layer123, and light emitted from the active layer 123 is absorbed, therebycausing loss of light. On the other hand, when the Al content in theetching stop layer 126 is set so that loss of light does not occur, itis not possible to ensure a sufficient Al content difference requiredfor a selective etching process, whereby the etching process proceeds tothe first p-type cladding layer 125 beyond the etching stop layer 126.

Thus, by using an AlGaInP-based semiconductor material for the etchingstop layer 126, the second p-type cladding layer 127 can be processedinto a stripe (ridge) shape with a good reproducibility and a highprecision.

Then, as illustrated in FIG. 18C, the first current blocking layer 129made of n-type AlInP and the second current blocking layer 130 made ofn-type GaAs are grown in this order by using an MOCVD method or an MBEmethod, for example, after which the silicon oxide film 135 is liftedoff so as to remove portions of the first current blocking layer 129 andthe second current blocking layer 130 that are located above the firstcontact layer 128.

Then, although not shown, the second contact layer 131 made of p-typeGaAs is grown on the first contact layer 128 and the second currentblocking layer 130, after which the n-side electrode 132 and the p-sideelectrode 133 are formed on the lower surface of the substrate 121 andon the upper surface of the second contact layer 131, respectively,thereby obtaining the semiconductor laser device of the eighthembodiment.

As described above, according to the eighth embodiment, the first bufferlayer 124 is provided between the optical guiding layer that is theoutermost layer of the active layer 123 and the first p-type claddinglayer 125, whereby the first p-type cladding layer 125 of AlGaAs can beformed, with a reduced crystal defect density, over the active layer 123made of an AlGaInP-based semiconductor material. Thus, the first p-typecladding layer 125 and the second p-type cladding layer 127 can beprovided with a high thermal conductivity, whereby heat generated in theactive layer 123 can be efficiently radiated into the heat sink. Thus,it is possible to realize, with a reduced defect density, an increase inthe output power of the semiconductor laser device. Specifically, thesemiconductor laser device of the eighth embodiment is capable ofoperating at an output power as high as 120 mW without saturating itsoutput power even at an environmental temperature of about 70° C.

Variation of Eighth Embodiment

One variation of the eighth embodiment will now be described withreference to the drawings.

FIG. 19 is a cross-sectional view illustrating the semiconductor laserdevice according to the variation of the eighth embodiment. In FIG. 19,like components to those of the semiconductor laser device of the eighthembodiment illustrated in FIG. 17 are denoted by like reference numeralsand will not be further described below.

As illustrated in FIG. 19, in the semiconductor laser device of thepresent variation, an etching stop layer 136 made of p-typeGa_(0.5)In_(0.5)P having a thickness of about 50 nm is formed on thefirst p-type cladding layer 125, instead of the etching stop layer 126made of p-type AlInP illustrated in FIG. 17, and a second buffer layer137 made of p-type Al_(0.5)In_(0.5)P having a thickness of about 1 nm isformed between the etching stop layer 136 and the second p-type claddinglayer 127. Note that the compound composition, the thickness and thedoping concentration of each of the semiconductor layers other than theetching stop layer 136 and the second buffer layer 137 are as shown inTable 4 above.

A feature of the present variation is that, in addition to the provisionof the first buffer layer 124, whereby the first p-type cladding layer125 made of AlGaAs can be formed with a reduced crystal defect densityas in the eighth embodiment, the second buffer layer 137 is providedbetween the etching stop layer 136 and the second p-type cladding layer127, whereby it is possible to reduce the crystal defect density at theinterface with the second p-type cladding layer 127 even if the Gacontent of the etching stop layer 136 is increased.

Note that the thickness of the second buffer layer 137 is not limited toabout 1 nm, but may alternatively be any other suitable value as long asit is 0.5 nm or more and 5 nm or less, and it is still possible toobtain the effect of reducing the crystal defect density withoutinfluencing the characteristics of the semiconductor laser device.

Specifically, in a case where the first p-type cladding layer 125 isformed directly on the etching stop layer 136 without using the secondbuffer layer 137, since the etching stop layer 136 has a relativelylarge Ga content of 0.5, the crystal defect density is 1×10⁷/cm² ormore. In contrast, in the present variation, the number of crystaldefects per square centimeter at the interface between the second bufferlayer 137 and the first p-type cladding layer 125 is on the order of 10,indicating that the device performance can be improved through thereduction of the crystal defect density.

Note that in the present variation, the compound composition of thesecond buffer layer 137 is not limited to Al_(0.5)In_(0.5)P, but mayalternatively be any other suitable composition as long as it is AlGaInPwhose Ga content is smaller than that in the etching stop layer 136. Forexample, when the Ga content is 0.15 or less, the crystal defect densityat the interface between the second buffer layer 137 and the etchingstop layer 136 is as low as 1×10⁵/cm² or less.

A method for manufacturing the semiconductor laser device of the presentvariation will now be described with reference to FIG. 18A to FIG. 18C.

First, as in the step illustrated in FIG. 18A, the n-type cladding layer122, the active layer 123, the first buffer layer 124, the first p-typecladding layer 125, the etching stop layer 136 made of p-type GaInP, asecond buffer layer formation layer made of p-type AlInP, the secondp-type cladding layer formation layer 127A, the first contact layerformation layer 128A, and the protection layer 134, are grown in thisorder on the substrate 121.

Then, as in the step illustrated in FIG. 18B, the protection layer 134is removed, after which the first contact layer formation layer 128A,the second p-type cladding layer formation layer 127A and the secondbuffer layer formation layer are successively etched while using thepatterned silicon oxide film 135 as a mask to form the first contactlayer 128, the second p-type cladding layer 127 and the second bufferlayer 137, respectively, which are stripe-shaped.

Herein, the second buffer layer formation layer can be etched by using,for example, sulfuric acid as an etchant. In this way, it can be etchedselectively with respect to the etching stop layer 136, whereby it ispossible to form the second buffer layer 137 with a goodcontrollability.

Then, as in the step illustrated in FIG. 18C, the first current blockinglayer 129, the second current blocking layer 130 and the second contactlayer 131 are grown in this order, after which the n-side electrode 132and the p-side electrode 133 are formed, thereby obtaining thesemiconductor laser device of the present variation.

The etching stop layer 126 of the eighth embodiment is made of AlInP andcontains Al. Therefore, the etching stop layer 126, which has beenexposed after etching the second p-type cladding layer 127, may beoxidized to deteriorate the crystallinity of the first current blockinglayer 129 and the second current blocking layer 130, which are formed onthe etching stop layer 126. In contrast, the etching stop layer 136 ofthe present variation is made of GaInP and does not contain Al.Therefore, it is possible to suppress the oxidization of the etchingstop layer 136, which has been exposed after etching the second bufferlayer 137, whereby it is possible to improve the crystalline quality ofthe first current blocking layer 129 and the second current blockinglayer 130.

As described above, according to the variation of the eighth embodiment,a high output power can be obtained as in the eighth embodiment. Inaddition, the second buffer layer 137 is provided, whereby even if theAl content in the etching stop layer 136 is reduced, the crystal defectdensity at the interface between the etching stop layer 136 and thesecond p-type cladding layer 127 does not increase. In this way, theoxidization of the etching stop layer 136 during the manufacturingprocess is suppressed, thereby improving the crystalline quality of thefirst current blocking layer 129 and the second current blocking layer130 and improving the reliability of the semiconductor laser device ascompared to that in the eighth embodiment.

Note that in the semiconductor laser devices of the eighth embodimentand the variation thereof it is more preferred that n-type AlGaAs isused as the compound semiconductor of the n-type cladding layer 122.Thus, an AlGaAs-based semiconductor material, which has a high thermalconductivity, is used for the n-type cladding layer 122, in addition tothe first p-type cladding layer 125 and the second p-type cladding layer127, whereby heat generated in the active layer 123 is more effectivelyradiated, and the output power can be further increased.

Moreover, in the semiconductor laser devices of the eighth embodimentand the variation thereof, the material of the substrate 121 is notlimited to n-type GaAs, but the substrate 121 may alternatively be ap-type substrate made of p-type GaAs, for example.

Moreover, in the semiconductor laser devices of the eighth embodimentand the variation thereof, a real refractive index waveguide is formedby using AlInP for the first current blocking layer 129. Alternatively,a complex refractive index waveguide may be formed by using GaAs for thefirst current blocking layer 129.

Moreover, in the semiconductor laser devices of the eighth embodimentand the variation thereof, the active layer 123 is not limited to thoseof a multiple quantum well structure, but may alternatively be an activelayer of a single quantum well structure or a single bulk active layer,for example.

Moreover, in the semiconductor laser devices of the eighth embodimentand the variation thereof, the waveguide structure is not limited to aridge-shaped waveguide structure, but may alternatively be any othersuitable structure such as an internal stripe-shaped waveguidestructure, and it is still possible to obtain similar effects.

Ninth Embodiment

A semiconductor laser device according to the ninth embodiment will nowbe described with reference to the drawings.

FIG. 20 is a cross-sectional view illustrating the semiconductor laserdevice according to the ninth embodiment of the present invention. Asillustrated in FIG. 20, an n-type cladding layer 142 made of n-typeAlGaInP, an active layer 143 of a multiple quantum well structureincluding a multiple quantum well and optical guiding layers interposingthe multiple quantum well therebetween, and a first p-type claddinglayer 144 made of p-type AlGaInP, are deposited in this order on asubstrate 141 made of n-type GaAs.

Note that in the active layer 143, the multiple quantum well has analternating pattern of three well layers made of GaInP and two barrierlayers made of AlGaInP, with two of the well layers being the outermostlayers of the multiple quantum well, and an optical guiding layer madeof AlGaInP is formed on the upper side and on the lower side of themultiple quantum well.

A second p-type cladding layer 145 made of p-type AlGaAs is formed in astripe shape on the first p-type cladding layer 144, and a first contactlayer 146 made of p-type GaInP is formed on the second p-type claddinglayer 145. Moreover, a first current blocking layer 147 made of p-typeAlInP and a second current blocking layer 148 made of n-type GaAs areformed in this order on the upper surface of the first p-type claddinglayer 144 beside the second p-type cladding layer 145 and on the sidesurface of the second p-type cladding layer 145. Moreover, a secondcontact layer 149 made of p-type GaAs is formed on the first contactlayer 146 and the second current blocking layer 148.

Moreover, an n-side electrode 150 is formed under the substrate 141. Then-side electrode 150 is made of an alloy containing Au, Ge and Ni, forexample, and is in ohmic contact with the substrate 141. Moreover, ap-side electrode 151 is formed on the second contact layer 149. Thep-side electrode 151 is made of an alloy containing Cr, Pt and Au, forexample, and is in ohmic contact with the second contact layer 149.

Herein, the structure is doped with Si as an n-type impurity and Zn as ap-type impurity. Moreover, the doping concentration is substantiallyequal to the carrier concentration at room temperature.

Table 5 below shows, as an example, the specific thickness, compoundcomposition and doping concentration of each of the semiconductorlayers.

TABLE 5 Compound Doping Thickness Composition concentration (cm⁻³)Semiconductor layer Second contact layer   3 μm GaAs p-type 2 × 10¹⁸Second current blocking layer  0.2 μm GaAs n-type 1 × 10¹⁸ First currentblocking layer  0.3 μm Al_(0.5)In_(0.5)P n-type 1 × 10¹⁸ First contactlayer   50 nm Ga_(0.5)In_(0.5)P p-type 1 × 10¹⁸ Second p-type claddinglayer   1 μm Al_(0.8)Ga_(0.2)As p-type 1 × 10¹⁸ First p-type claddinglayer  0.2 μm Al_(0.4)Ga_(0.1)In_(0.5)P p-type 5 × 10¹⁷ Active layerMultiple quantum well Well layers   6 nm each Ga_(0.5)In_(0.5)P — —Barrier layers   5 nm each Al_(0.25)Ga_(0.25)In_(0.5)P — — Opticalguiding layers   20 nm each Al_(0.25)Ga_(0.25)In_(0.5)P — — N-typecladding layer   2 μm Al_(0.35)Ga_(0.15)In_(0.5)P n-type 1 × 10¹⁸Substrate  100 μm GaAs n-type 1 × 10¹⁸

As shown in Table 5 above, the Ga content in the first p-type claddinglayer 144 is 0.1, so that in the layered structure in which the secondp-type cladding layer 145 made of AlGaAs is deposited on the firstp-type cladding layer 144 made of AlGaInP, the Ga content of thesemiconductor layer made of AlGaInP is relatively small.

Moreover, in the ninth embodiment, the In content, 1-x-y, in eachsemiconductor layer made of Al_(x)Ga_(y)In_(1-x-y)P is set to be 0.5 forthe lattice match with the substrate 141 made of GaAs. Note however thatfor the purpose of realizing a lattice match with the substrate 141 madeof GaAs, the In content, 1-x-y, in each semiconductor layer made ofAl_(x)Ga_(y)In_(1-x-y)P is not limited to 0.5, but may alternatively beany other suitable value as long as it satisfies 0.45≦1-x-y≦0.55.

As in the semiconductor laser device of the eighth embodiment, in thesemiconductor laser device of the ninth embodiment, the active layer 143has a multiple quantum well structure with a band gap corresponding to awavelength of 650 nm. Therefore, as the current passes through the gapin the first current blocking layer 147 and the second current blockinglayer 148 and reaches the active layer 143, laser light is emitted withan oscillation wavelength of 650 nm.

Herein, in the semiconductor laser device of the ninth embodiment, anAlGaAs-based semiconductor material, which has a high thermalconductivity, is used for the second p-type cladding layer 145.Therefore, heat generated in the active layer 143 can be efficientlyradiated into a heat sink (not shown) to suppress the increase in thetemperature of the active layer 143.

Moreover, since an AlGaInP-based semiconductor material having a largeband gap is used for the first p-type cladding layer 144, it is possibleto provide a large band gap difference between the active layer 143 andthe first p-type cladding layer 144, whereby it is possible to suppressthe overflow of electrons from the active layer 143 into the firstp-type cladding layer 144.

Thus, by using an AlGaInP-based semiconductor material for the firstp-type cladding layer 144, electrons injected into the active layer 143efficiently contribute to the radiative recombination. Moreover, byusing an AlGaAs-based semiconductor material for the second p-typecladding layer 145, heat generated in the active layer 143 can beradiated into the heat sink, thereby suppressing the temperatureincrease. In this way, even if the current value is increased in orderto increase the output power of the device, the light-emittingefficiency does not decrease due to thermal saturation. Thus, it ispossible to increase the output power of the semiconductor laser device.Specifically, the semiconductor laser device of the ninth embodiment iscapable of operating at an output power as high as 120 mW withoutsaturating its output power even at an environmental temperature ofabout 70° C.

Furthermore, in the layered structure in which the second p-typecladding layer 145 made of AlGaAs is deposited on the first p-typecladding layer 144 made of AlGaInP, the first p-type cladding layer 144has a relatively small Ga content of 0.1, whereby it is possible toreduce the crystal defect density at the interface between the firstp-type cladding layer 144 and the second p-type cladding layer 145.Specifically, the crystal defect density at the interface between thefirst p-type cladding layer 144 and the second p-type cladding layer 145is 1×10⁴/cm² or less, indicating that the device performance can beimproved through the reduction of the crystal defect density.

As described above, in the semiconductor laser device of the ninthembodiment, the second p-type cladding layer 145 made of AlGaAs isprovided on the first p-type cladding layer 144 made of AlGaInP in orderto suppress the overflow of electrons from the active layer 143 into thefirst p-type cladding layer 144 and to increase the output power byefficiently radiating heat generated in the active layer 143, whereinthe first p-type cladding layer 144 has a small Ga content, whereby itis possible to reduce the crystal defect density at the interfacebetween the first p-type cladding layer 144 and the second p-typecladding layer 145.

First Variation of Ninth Embodiment

A semiconductor laser device according to the first variation of theninth embodiment will now be described with reference to the drawings.

FIG. 21 is a cross-sectional view illustrating a semiconductor laserdevice according to the first variation of the ninth embodiment. In FIG.21, like components to those of the semiconductor laser device of theninth embodiment illustrated in FIG. 20 are denoted by like referencenumerals and will not be further described below.

As illustrated in FIG. 21, the semiconductor laser device of the presentvariation differs from the semiconductor laser device of the ninthembodiment in that a buffer layer 152 made of p-typeAl_(0.45)Ga_(0.05)In_(0.5)P having a thickness of about 1 nm is providedbetween the first p-type cladding layer 144 and the second p-typecladding layer 145.

Herein, the Ga content in the buffer layer 152 is smaller than that inthe first p-type cladding layer 144. Thus, in the semiconductor laserdevice of the present variation, the layered structure including thefirst p-type cladding layer 144, the buffer layer 152 and the secondp-type cladding layer 145, which are deposited in this order, is similarto the group III-V compound semiconductor layered structure of the firstembodiment. Therefore, it is possible to reduce the crystal defectdensity at the interface between the buffer layer 152 and the secondp-type cladding layer 145 as compared to the semiconductor laser deviceof the ninth embodiment in which the second p-type cladding layer 145 isformed directly on the first p-type cladding layer 144.

Note that the thickness of the buffer layer 152 is not limited to about1 nm, but may alternatively be any other suitable value as long as it is0.5 nm or more and 5 nm or less, and it is still possible to obtain theeffect of reducing the crystal defect density without influencing thecharacteristics of the semiconductor laser device.

Thus, in the semiconductor laser device of the present variation, thecrystal defect density at the interface between the buffer layer 152 andthe second p-type cladding layer 145 is determined by the compoundcomposition of the buffer layer 152 and is not dependent on the compoundcomposition of the first p-type cladding layer 144. Therefore, it ispossible to ensure sufficient freedom in designing the compoundcomposition of the first p-type cladding layer 144. Particularly, it ispossible to reduce the crystal defect density without influencing thevarious characteristics of the semiconductor laser device such as theoperating current and the divergence angle in the vertical direction,which are determined by the Al content of the cladding layer.

Second Variation of Ninth Embodiment

A semiconductor laser device according to the second variation of theninth embodiment will now be described with reference to the drawings.

FIG. 22 is a cross-sectional view illustrating the semiconductor laserdevice according to the second variation of the ninth embodiment. InFIG. 22, like components to those of the semiconductor laser device ofthe ninth embodiment illustrated in FIG. 20 are denoted by likereference numerals and will not be further described below.

As illustrated in FIG. 22, the semiconductor laser device of the presentvariation differs from the semiconductor laser device of the ninthembodiment in that an etching stop layer 153 made of Ga_(0.5)In_(0.5)Phaving a thickness of about 10 mm is formed on the first p-type claddinglayer 144, and a buffer layer 154 made of p-type Al_(0.5)In_(0.5)Phaving a thickness of about 1 nm is provided between the etching stoplayer 153 and the second p-type cladding layer 145.

Herein, the Ga content in the buffer layer 154 is smaller than that inthe etching stop layer 153. Thus, in the semiconductor laser device ofthe present variation, the layered structure including the etching stoplayer 153, the buffer layer 154 and the second p-type cladding layer145, which are deposited in this order, is similar to the group III-Vcompound semiconductor layered structure of the first embodiment.Therefore, in a case where the etching stop layer 153 is made of GaInP,it is possible to reduce the crystal defect density at the interfacebetween the buffer layer 154 and the second p-type cladding layer 145 ascompared to a case where the second p-type cladding layer 145 is formeddirectly on the etching stop layer 153.

A method for manufacturing the semiconductor laser device of the presentvariation will now be described.

The semiconductor laser device of the present variation can bemanufactured by a method similar to that for the semiconductor laserdevice of the eighth embodiment illustrated in FIG. 18A to FIG. 18C.

Specifically, the n-type cladding layer 142, the active layer 143, thefirst p-type cladding layer 144, the etching stop layer 153, a bufferlayer formation layer made of p-type AlInP, a second p-type claddinglayer formation layer made of p-type AlGaAs, and a first contact layerformation layer made of a p-type GaInP, are grown in this order on thesubstrate 141. Then, the first contact layer formation layer, the secondp-type cladding layer formation layer and the buffer layer formationlayer are selectively etched in this order while using a silicon oxidefilm that has been patterned into a stripe shape as a mask, therebyforming the first contact layer 146, the second p-type cladding layer145 and the buffer layer 154, respectively. Then, the first currentblocking layer 147, the second current blocking layer 148 and the secondcontact layer 149 are grown in this order, after which the n-sideelectrode 150 and the p-side electrode 151 are formed, thereby obtainingthe semiconductor laser device of the present variation.

Note that the first contact layer formation layer made of GaInP can beetched by using a hydrochloric acid-type etchant. In this way, the firstcontact layer formation layer can be etched selectively with respect tothe second p-type cladding layer formation layer made of AlGaAs.Similarly, the second p-type cladding layer formation layer can beetched by using a mixture of ammonia and hydrogen peroxide, or the like,and the buffer layer formation layer can be etched by using sulfuricacid, or the like. In this way, each of these layers can be etchedselectively. Thus, it is possible to precisely form the buffer layer154, the second p-type cladding layer 145 and the first p-type claddinglayer, which are stripe-shaped.

Herein, although the etching stop layer 153 is exposed after forming thebuffer layer 154 by etching, the etching stop layer 153 is made of GaInPand thus does not contain Al, whereby the oxidization of the etchingstop layer 153 is suppressed, and it is possible to improve thecrystalline quality of the first current blocking layer 147 and thesecond current blocking layer 148.

Thus, in the semiconductor laser device of the present variation, theetching stop layer 153 is provided between the first p-type claddinglayer 144 and the second p-type cladding layer 145, and the Al contentof the etching stop layer 153 is reduced, so as to prevent the etchingstop layer 153 from being oxidized during the step of precisely formingthe second p-type cladding layer 145, wherein the buffer layer 154 isprovided between the etching stop layer 153 and the second p-typecladding layer 145. In this way, it is possible to reduce the crystaldefect density in the second p-type cladding layer 145.

Note that in the semiconductor laser devices of the ninth embodiment andthe variations thereof it is preferred that the n-type cladding layer142 is made of n-type AlGaAs. In this way, in addition to the secondp-type cladding layer 145, the n-type cladding layer 142 is also made ofan AlGaAs-based semiconductor material, which has a high thermalconductivity, whereby heat generated in the active layer 143 can beeffectively radiated, allowing the output power to be further increased.

Moreover, in the semiconductor laser devices of the ninth embodiment andthe variations thereof the material of the substrate 141 is not limitedto n-type GaAs, but the substrate 141 may alternatively be a p-typesubstrate made of p-type GaAs, for example.

Moreover, in the semiconductor laser devices of the ninth embodiment andthe variations thereof, a real refractive index waveguide is formed byusing AlInP for the first current blocking layer 147. Alternatively, acomplex refractive index waveguide may be formed by using GaAs for thefirst current blocking layer 147.

Moreover, in the semiconductor laser devices of the ninth embodiment andthe variations thereof, the active layer 143 is not limited to those ofa multiple quantum well structure, but may alternatively be an activelayer of a single quantum well structure or a single bulk active layer,for example.

Moreover, in the semiconductor laser devices of the ninth embodiment andthe variations thereof, the waveguide structure is not limited to aridge-shaped waveguide structure, but may alternatively be any othersuitable structure such as an internal stripe-shaped waveguidestructure, and it is still possible to obtain similar effects.

Tenth Embodiment

A semiconductor device according to the tenth embodiment will now bedescribed with reference to the drawings.

FIG. 23 is a cross-sectional view illustrating the semiconductor deviceaccording to the tenth embodiment. As illustrated in FIG. 23, thesemiconductor device of the tenth embodiment is a heterojunction bipolartransistor (HBT), in which a collector layer 162 made of n-type AlGaInP,a buffer layer 163 made of n-type AlGaInP, a base layer 164 made ofp-type AlGaAs, and an emitter layer 165 made of n-type AlGaAs, aredeposited in this order on a substrate 161 made of undoped GaAs.

The collector layer 162, the base layer 164 and the emitter layer 165are formed in a stair shape so that at least a portion of the uppersurface of each of these layers is exposed. A collector electrode 167 isformed on the exposed portion of the upper surface of the collectorlayer 162. Similarly, a base electrode 168 and an emitter electrode 169are formed on the base layer 164 and the emitter layer 165,respectively.

Herein, the collector layer 162, the buffer layer 163 and the emitterlayer 165 are doped with Si as an n-type impurity, and the base layer164 is doped with Zn as a p-type impurity.

Table 6 below shows, as an example, the specific thickness, compoundcomposition and doping concentration of each of the semiconductorlayers.

TABLE 6 Compound Doping Semiconductor layer Thickness Compositionconcentration (cm⁻³) Emitter layer  0.5 μm Al_(0.5)Ga_(0.5)As n-type 1 ×10¹⁸ Base layer  0.5 μm Al_(0.2)Ga_(0.8)As p-type 3 × 10¹⁷ Buffer layer  1 nm Al_(0.25)Ga_(0.25)In_(0.5)P n-type 1 × 10¹⁸ Collector layer   1μm Al_(0.35)Ga_(0.15)In_(0.5)P n-type 1 × 10¹⁸ Substrate  100 μm GaAs ——

As shown in Table 6 above, the Ga content in the buffer layer 163 issmaller than that in the collector layer 162. Thus, in the semiconductordevice of the tenth embodiment, the layered structure including thecollector layer 162, the buffer layer 163 and the base layer 164, whichare deposited in this order, is similar to the group III-V compoundsemiconductor layered structure of the first embodiment.

In the tenth embodiment, the buffer layer 163 is provided, whereby ascompared to a case where the base layer 164 is formed directly on thecollector layer 162, the crystal defect density in the base layer 164can be reduced, thus improving the current characteristic of the HBT.

Eleventh Embodiment

A semiconductor device according to the eleventh embodiment will now bedescribed with reference to the drawings.

FIG. 24 is a cross-sectional view illustrating the semiconductor deviceaccording to the eleventh embodiment. As illustrated in FIG. 24, thesemiconductor device of the eleventh embodiment is a field effecttransistor (FET), in which a blocking layer 172 made of undoped AlGaInP,a buffer layer 173 made of undoped AlGaInP, a channel layer 174 made ofn-type AlGaAs, and a current supply layer 175 made of n-type AlGaAs, aredeposited in this order on a substrate 171 made of undoped GaAs.

A gate electrode 177 is formed on the current supply layer 175 via agate insulating film 176 therebetween, and a source electrode 178 and adrain electrode 179 are formed so as to be in ohmic contact with thecurrent supply layer 175.

Table 7 below shows, as an example, the specific thickness, compoundcomposition and doping concentration of each of the semiconductorlayers.

TABLE 7 Compound Doping Semiconductor layer Thickness Compositionconcentration (cm⁻³) Electron supply  0.5 μm Al_(0.5)Ga_(0.5)As n-type 1× 10¹⁸ layer Channel layer  0.5 μm Al_(0.2)Ga_(0.8)As n-type 1 × 10¹⁸Buffer layer   1 nm Al_(0.25)Ga_(0.25)In_(0.5)P — — Blocking layer   1μm Al_(0.35)Ga_(0.15)In_(0.5)P — — Substrate  100 μm GaAs — —

As shown in Table 7 above, the Ga content in the buffer layer 173 issmaller than that in the blocking layer 172. Thus, in the semiconductordevice of the eleventh embodiment, the layered structure including theblocking layer 172, the buffer layer 173 and the channel layer 174,which are deposited in this order, is similar to the group III-Vcompound semiconductor layered structure of the first embodiment.

According to the eleventh embodiment, the buffer layer 173 is provided,whereby as compared to a case where the channel layer 174 is formeddirectly on the blocking layer 172, the crystallinity of the channellayer 174 is improved. Thus, as compared to a case where the channellayer 174 is formed directly on the blocking layer 172, the crystaldefect density in the channel layer 174 can be reduced, whereby atwo-dimensional electron gas can be more easily formed at the interfacewith the channel layer, thus improving the high frequency characteristicof the FET.

As described above, in the method for manufacturing a group III-Vcompound semiconductor device of the present invention, a secondsemiconductor layer made of AlGaAs is formed on a first semiconductorlayer made of AlGaInP, while the Ga content of the first semiconductorlayer or the Al content of the second semiconductor layer is reduced,whereby it is possible to reduce the crystal defect density at theinterface between the first semiconductor layer and the secondsemiconductor layer.

Moreover, without modifying the compound composition of the firstsemiconductor layer and that of the second semiconductor layer, a bufferlayer made of AlGaInP whose Ga content is smaller than that of the firstsemiconductor layer or AlGaAs whose Al content is smaller than that ofthe second semiconductor layer may be formed between the firstsemiconductor layer and the second semiconductor layer, whereby it ispossible to obtain a group III-V compound semiconductor layeredstructure with a reduced crystal defect density while obtainingsubstantially the same properties as those of a layered structure inwhich the second semiconductor layer is deposited directly on the firstsemiconductor layer. In this way, it is possible to improve theperformance of various group III-V compound semiconductor devices asshown in the third to eleventh embodiments above.

1. A group III-V compound semiconductor device, comprising: a firstsemiconductor layer made of Al_(a)Ga_(b)In_(1-a-b)P (where 0≦a≦1, 0≦b≦1,0≦a+b≦1); a second semiconductor layer made of Al_(x)Ga_(y)In_(1-x-y)P(where 0≦x≦1, 0≦y≦1, 0≦x+y≦1) formed on the first semiconductor layer;and a third semiconductor layer made of Al_(c)Ga_(1-c)As (where 0≦c≦1)formed in contact with the second semiconductor layer, wherein a Gacontent y in the second semiconductor layer is smaller than a Ga contentb in the first semiconductor layer, the second semiconductor layer isprovided between the first semiconductor layer and the thirdsemiconductor layer, and the Ga content y is less than 0.15.
 2. Thegroup III-V compound semiconductor device of claim 1, furthercomprising: a first cladding layer made of a compound semiconductor of afirst conductivity type formed on a substrate; the third semiconductorlayer is provided on the first cladding layer; an active layer formed onthe third semiconductor layer; and a second cladding layer made of acompound semiconductor of a second conductivity type formed on theactive layer, wherein: the active layer is a layered structure includinga plurality of semiconductor layers; and the first semiconductor layeris provided so as to form an uppermost layer of the active layer.
 3. Thegroup III-V compound semiconductor device of claim 1, furthercomprising: a first cladding layer made of a compound semiconductor of afirst conductivity type formed on a substrate; an active layer formed onthe first cladding layer; and a second cladding layer made of a compoundsemiconductor of a second conductivity type formed on the active layer,wherein the first semiconductor layer is provided in a stripe shape onthe second cladding layer.
 4. The group III-V compound semiconductordevice of claim 3, further comprising: a first contact layer made ofAl_(d)Ga_(1-d)As (where 0≦d≦1) of the second conductivity type formed onthe third semiconductor layer; and a second contact layer made of GaAsof the second conductivity type formed on the first contact layer,wherein an Al content d in the first contact layer is larger than an Alcontent c in the third semiconductor layer.
 5. The group III-V compoundsemiconductor device of claim 1, further comprising: a first claddinglayer made of a compound semiconductor of a first conductivity typeformed on a substrate; an active layer formed on the first claddinglayer; and a second cladding layer made of a compound semiconductor ofthe second conductivity type formed in a stripe shape on the activelayer, wherein the first semiconductor layer is provided on the secondcladding layer.
 6. The group III-V compound semiconductor device ofclaim 1, further comprising: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; and anactive layer formed on the first cladding layer, wherein the firstsemiconductor layer is provided on the active layer.
 7. The group III-Vcompound semiconductor device of claim 6, wherein: the firstsemiconductor layer includes a stripe-shaped protrusion; the secondsemiconductor layer is provided at a foot of the protrusion and on aside surface of the protrusion; and the third semiconductor layercontains an impurity of the first conductivity type and is provided withan opening corresponding to a top surface of the protrusion so that thethird semiconductor layer covers the second semiconductor layer exceptfor an area over the top surface of the protrusion.
 8. The group III-Vcompound semiconductor device of claim 7, wherein the secondsemiconductor layer contains an impurity of the first conductivity type.9. The group III-V compound semiconductor device of claim 7, wherein thesecond semiconductor layer is provided on the first semiconductor layerso as to cover the first semiconductor layer including the top surfaceof the protrusion.
 10. The group III-V compound semiconductor device ofclaim 9, wherein the second semiconductor layer contains an impurity ofthe second conductivity type.
 11. The group III-V compound semiconductordevice of claim 1, further comprising: a first cladding layer made of acompound semiconductor of a first conductivity type formed on asubstrate; an active layer formed on the first cladding layer; and asecond cladding layer made of a compound semiconductor of a secondconductivity type formed on the active layer, wherein: the firstsemiconductor layer includes a stripe-shaped opening and contains animpurity of the first conductivity type; and the second semiconductorlayer is provided on a wall surface of the opening in the firstsemiconductor layer and on an upper surface of the first semiconductorlayer.
 12. The group III-V compound semiconductor device of claim 1,further comprising: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; and anactive layer formed on the first cladding layer, wherein: the activelayer is a layered structure including a plurality of semiconductorlayers; and the first semiconductor layer is provided so as to form anuppermost layer of the active layer.
 13. The group III-V compoundsemiconductor device of claim 12, further comprising: an etching stoplayer made of Al_(d)Ga_(e)In_(1-d-e)P (where 0≦d1, 0≦e≦1, 0≦d+e≦1)formed on the third semiconductor layer; and a second cladding layermade of Al_(f)Ga_(1-f)As (where 0≦f≦1) of a second conductivity typeformed on the etching stop layer, wherein a Ga content e in the etchingstop layer is 0.35 or less.
 14. The group III-V compound semiconductordevice of claim 12, further comprising: an etching stop layer made ofAl_(d)Ga_(e)In_(1-d-e)P (where 0≦d≦1, 0≦e≦1, 0d+e≦1) formed on the thirdsemiconductor layer; a fourth semiconductor layer made ofAl_(f)Ga_(g)In_(1-f-g)P (where 0≦f≦1, 0≦g≦1, 0≦f+g≦1) formed on theetching stop layer; and a second cladding layer made ofAl_(h)Gal_(1-h)As (where 0≦h≦1) of a second conductivity type formed onthe fourth semiconductor layer, wherein a Ga content g in the fourthsemiconductor layer is smaller than a Ga content e in the etching stoplayer.
 15. The group III-V compound semiconductor device of claim 12,wherein the first cladding layer is made of Al_(d)Ga_(1-d)As (where0≦d≦1).
 16. The group III-V compound semiconductor device of claim 1,further comprising: a first cladding layer made of a compoundsemiconductor of a first conductivity type formed on a substrate; and anactive layer formed on the first cladding layer, wherein the firstsemiconductor layer is formed on the active layer.
 17. The group III-Vcompound semiconductor device of claim 1, further comprising: a firstcladding layer made of a compound semiconductor of a first conductivitytype formed on a substrate; an active layer formed on the first claddinglayer; and a second cladding layer made of Al_(d)Ga_(e)In_(1-d-e)P(where 0≦d≦1, 0≦e≦1, 0≦d+e≦1) of a second conductivity type formed onthe active layer, wherein the first semiconductor layer is formed on thesecond cladding layer.
 18. The group III-V compound semiconductor deviceof claim 1, wherein a Ga content y in the second semiconductor layer iszero.
 19. The group III-V compound semiconductor device of claim 1,wherein: the substrate is made of GaAs; an In content 1-a-b in the firstsemiconductor layer satisfies 0.45≦1-a-b≦0.55; and an In content 1-x-yin the second semiconductor layer satisfies 0.45≦1-x-y≦0.55.
 20. Thegroup III-V compound semiconductor device of claim 1, wherein athickness of the second semiconductor layer is 0.5 nm or more and 5 nmor less.
 21. A group III-V compound semiconductor device, comprising: afirst semiconductor layer made of Al_(a)Ga_(b)In_(1-a-b)P (where 0≦a≦1,0≦b≦1, 0≦a+b≦1); a second semiconductor layer made of Al_(z)Ga_(1-z)As(where 0≦z≦1) formed on the first semiconductor layer; and a thirdsemiconductor layer made of Al_(c)Ga_(1-c)As (where 0≦c≦1) formed incontact with the second semiconductor layer, wherein the secondsemiconductor layer is provided between the first semiconductor layerand the third semiconductor layer, an Al content z in the secondsemiconductor layer is smaller than an Al content c in the thirdsemiconductor layer, and an Al content c in the third semiconductorlayer is 0.3 or less.
 22. The group III-V compound semiconductor deviceof claim 21, further comprising an active layer formed on a substrate,the active layer being a layered structure including a plurality ofsemiconductor layers, wherein: the first semiconductor layer is formedon the substrate; and the third semiconductor layer is provided so as toform a lowermost layer of the active layer.
 23. The group III-V compoundsemiconductor device of claim 21, wherein an Al content z in the secondsemiconductor layer is zero.
 24. The group III-V compound semiconductordevice of claim 21, wherein: the substrate is made of GaAs; and an Incontent 1-a-b in the first semiconductor layer satisfies0.45≦1-a-b≦0.55.
 25. The group III-V compound semiconductor device ofclaim 21, wherein a thickness of the second semiconductor layer is 0.5nm or more and 5 nm or less.